ndb: make NostrEvents immutable

Since we can't mutate NdbNotes, let's update the existing codebase to
generate and sign ids on NostrEvent constructions. This will allow us to
match NdbNote's constructor

damus/Models/ProfileModel.swift

@@ -35,7 +35,7 @@ class ProfileModel: ObservableObject, Equatable {
         }
         
         for tag in contacts.tags {
-            guard tag.count >= 2 && tag[0] == "p" else {
+            guard tag.count >= 2 && tag[0].matches_char("p") else {
                 continue
             }