Add experimental push notification support

I added support for the experimental push notifications feature. There are many improvements to be made, so this feature is currently opt-in only. If the user does not opt-in, their device tokens will not be sent out and thus they will receive no push notifications.

We should perform more testing on real-life staging environments before fully releasing this feature.

Testing
-------

Testing was done gradually during development.

Device: iOS simulators
iOS: 17
Damus version: A few different but recent prototypes
Rough coverage:
1. Checked that no device tokens are sent out when setting is off
2. Checked that I can successfully receive device tokens when feature is ON and set to localhost.
3. Checked sending test push notifications of types "note" (kind: 1), reaction (kind: 7) and DMs (kind 4) works and shows a generic but reasonable push notification message
4. Checked that clicking on the notifications above take the user to the correct screen

Closes: https://github.com/damus-io/damus/issues/67
Changelog-Added: Add experimental push notification support
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Signed-off-by: William Casarin <jb55@jb55.com>

DamusNotificationService/DamusNotificationService.entitlements

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>com.apple.security.app-sandbox</key>
+	<true/>
+	<key>com.apple.security.application-groups</key>
+	<array>
+		<string>group.com.damus</string>
+	</array>
+	<key>com.apple.security.network.client</key>
+	<true/>
+</dict>
+</plist>

DamusNotificationService/Info.plist

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>NSExtension</key>
+	<dict>
+		<key>NSExtensionPointIdentifier</key>
+		<string>com.apple.usernotifications.service</string>
+		<key>NSExtensionPrincipalClass</key>
+		<string>$(PRODUCT_MODULE_NAME).NotificationService</string>
+	</dict>
+</dict>
+</plist>

DamusNotificationService/NostrEventInfoFromPushNotification.swift

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+//
+//  NostrEventInfoFromPushNotification.swift
+//  DamusNotificationService
+//
+//  Created by Daniel D’Aquino on 2023-11-13.
+//
+
+import Foundation
+
+/// The representation of a JSON-encoded Nostr Event used by the push notification server
+/// Needs to match with https://gitlab.com/soapbox-pub/strfry-policies/-/raw/433459d8084d1f2d6500fdf916f22caa3b4d7be5/src/types.ts
+struct NostrEventInfoFromPushNotification: Codable {
+    let id: String          // Hex-encoded
+    let sig: String         // Hex-encoded
+    let kind: NostrKind
+    let tags: [[String]]
+    let pubkey: String      // Hex-encoded
+    let content: String
+    let created_at: Int
+    
+    static func from(dictionary: [AnyHashable: Any]) -> NostrEventInfoFromPushNotification? {
+        guard let id = dictionary["id"] as? String,
+              let sig = dictionary["sig"] as? String,
+              let kind_int = dictionary["kind"] as? UInt32,
+              let kind = NostrKind(rawValue: kind_int),
+              let tags = dictionary["tags"] as? [[String]],
+              let pubkey = dictionary["pubkey"] as? String,
+              let content = dictionary["content"] as? String,
+              let created_at = dictionary["created_at"] as? Int else {
+            return nil
+        }
+        return NostrEventInfoFromPushNotification(id: id, sig: sig, kind: kind, tags: tags, pubkey: pubkey, content: content, created_at: created_at)
+    }
+    
+    func reactionEmoji() -> String? {
+        guard self.kind == NostrKind.like else {
+            return nil
+        }
+
+        switch self.content {
+        case "", "+":
+            return "❤️"
+        case "-":
+            return "👎"
+        default:
+            return self.content
+        }
+    }
+}

DamusNotificationService/NotificationFormatter.swift

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+//
+//  NotificationFormatter.swift
+//  DamusNotificationService
+//
+//  Created by Daniel D’Aquino on 2023-11-13.
+//
+
+import Foundation
+import UserNotifications
+
+struct NotificationFormatter {
+    static var shared = NotificationFormatter()
+    
+    // TODO: These is a very generic notification formatter. Once we integrate NostrDB into the extension, we should reuse various functions present in `HomeModel.swift`
+    func formatMessage(event: NostrEventInfoFromPushNotification) -> UNNotificationContent? {
+        let content = UNMutableNotificationContent()
+        if let event_json_data = try? JSONEncoder().encode(event),  // Must be encoded, as the notification completion handler requires this object to conform to `NSSecureCoding`
+           let event_json_string = String(data: event_json_data, encoding: .utf8) {
+            content.userInfo = [
+                "nostr_event_info": event_json_string
+            ]
+        }
+        switch event.kind {
+            case .text:
+                content.title = NSLocalizedString("Someone posted a note", comment: "Title label for push notification where someone posted a note")
+                content.body = event.content
+                break
+            case .dm:
+                content.title = NSLocalizedString("New message", comment: "Title label for push notifications where a direct message was sent to the user")
+                content.body = NSLocalizedString("(Contents are encrypted)", comment: "Label on push notification indicating that the contents of the message are encrypted")
+                break
+            case .like:
+                guard let reactionEmoji = event.reactionEmoji() else {
+                    content.title = NSLocalizedString("Someone reacted to your note", comment: "Generic title label for push notifications where someone reacted to the user's post")
+                    break
+                }
+                content.title = NSLocalizedString("New note reaction", comment: "Title label for push notifications where someone reacted to the user's post with a specific emoji")
+                content.body = String(format: NSLocalizedString("Someone reacted to your note with %@", comment: "Body label for push notifications where someone reacted to the user's post with a specific emoji"), reactionEmoji)
+                break
+            case .zap:
+                content.title = NSLocalizedString("Someone zapped you ⚡️", comment: "Title label for a push notification where someone zapped the user")
+                break
+            default:
+                return nil
+        }
+        return content
+    }
+}

DamusNotificationService/NotificationService.swift

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+//
+//  NotificationService.swift
+//  DamusNotificationService
+//
+//  Created by Daniel D’Aquino on 2023-11-10.
+//
+
+import UserNotifications
+import Foundation
+
+class NotificationService: UNNotificationServiceExtension {
+
+    var contentHandler: ((UNNotificationContent) -> Void)?
+    var bestAttemptContent: UNMutableNotificationContent?
+
+    override func didReceive(_ request: UNNotificationRequest, withContentHandler contentHandler: @escaping (UNNotificationContent) -> Void) {
+        self.contentHandler = contentHandler
+        
+        // Modify the notification content here...
+        guard let nostrEventInfoDictionary = request.content.userInfo["nostr_event"] as? [AnyHashable: Any],
+              let nostrEventInfo = NostrEventInfoFromPushNotification.from(dictionary: nostrEventInfoDictionary) else {
+            contentHandler(request.content)
+            return;
+        }
+        
+        if let improvedContent = NotificationFormatter.shared.formatMessage(event: nostrEventInfo) {
+            contentHandler(improvedContent)
+        }
+    }
+    
+    override func serviceExtensionTimeWillExpire() {
+        // Called just before the extension will be terminated by the system.
+        // Use this as an opportunity to deliver your "best attempt" at modified content, otherwise the original push payload will be used.
+        if let contentHandler = contentHandler, let bestAttemptContent =  bestAttemptContent {
+            contentHandler(bestAttemptContent)
+        }
+    }
+
+}