Improve notification view filtering UX

- Add subtitle below the toolbar title to indicate the state of the filter
- Add settings icon to take user to the notification settings page, and
  thus make that more discoverable

Testing
-------

PASS

Device: iPhone 13 mini
iOS: 17.6.1
Coverage:
1. Switching back and forth between the notifications tab and other tabs
   causes subtitle to show/hide as expected in both filter options
   (all, friends)
2. Subtitle follows the friends filter
3. Subtitle shows after restarting the app
4. Settings icon appears and takes user to the notification setting view
5. Notification settings can be updated from that view.

Changelog-Changed: Improve notification view filtering UX
Closes: https://github.com/damus-io/damus/issues/2480
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/Models/FriendFilter.swift

@@ -9,7 +9,7 @@ import Foundation
 
 enum FriendFilter: String, StringCodable {
     case all
-    case friends
+    case friends_of_friends
     
     init?(from string: String) {
         guard let ff = FriendFilter(rawValue: string) else {
@@ -27,8 +27,17 @@ enum FriendFilter: String, StringCodable {
         switch self {
         case .all:
             return true
-        case .friends:
+        case .friends_of_friends:
             return contacts.is_in_friendosphere(pubkey)
         }
     }
+    
+    func description() -> String {
+        switch self {
+            case .all:
+                return NSLocalizedString("All", comment: "Human-readable short description of the 'friends filter' when it is set to 'all'")
+            case .friends_of_friends:
+                return NSLocalizedString("Friends of friends", comment: "Human-readable short description of the 'friends filter' when it is set to 'friends-of-friends'")
+        }
+    }
 }