refactor: add Pubkey, Privkey, NoteId string aliases

This is a non-behavioral change in preparation for the actual switchover
from Strings to Ids. The purpose of this kit is to reduce the size of
the switchover commit which is going to be very large.

damus/Models/DirectMessageModel.swift

@@ -16,11 +16,11 @@ class DirectMessageModel: ObservableObject {
 
     @Published var draft: String = ""
     
-    let pubkey: String
-    
+    let pubkey: Pubkey
+
     var is_request = false
-    var our_pubkey: String
-    
+    var our_pubkey: Pubkey
+
     func determine_is_request() -> Bool {
         for event in events {
             if event.pubkey == our_pubkey {
@@ -31,7 +31,7 @@ class DirectMessageModel: ObservableObject {
         return true
     }
     
-    init(events: [NostrEvent] = [], our_pubkey: String, pubkey: String) {
+    init(events: [NostrEvent] = [], our_pubkey: Pubkey, pubkey: Pubkey) {
         self.events = events
         self.our_pubkey = our_pubkey
         self.pubkey = pubkey