contacts: get followed hashtags function

todo: cache these

damus/Models/Contacts.swift

@@ -70,6 +70,15 @@ class Contacts {
         return Array(friends)
     }
 
+    func get_followed_hashtags() -> Set<String> {
+        guard let ev = self.event else { return Set() }
+        return ev.tags.reduce(into: Set<String>(), { htags, tag in
+            if tag.count >= 2 && tag[0] == "t" && tag[1] != "" {
+                htags.insert(tag[1])
+            }
+        })
+    }
+
     func add_friend_pubkey(_ pubkey: String) {
         friends.insert(pubkey)
     }