mute: adding new structs/enums for new mute list

- Adding MuteItem & DamusDuration
- Changing RefId hashtag associated type from TagElem to Hashtag
    - This is done because in MuteItem, we can not create a RefId.hashtag TagElem instance since we don’t have a note associated with a given hashtag mute item.

Related: https://github.com/damus-io/damus/issues/1718
Related: https://github.com/damus-io/damus/issues/856
Lighting Address: fishcharlie@strike.me

Signed-off-by: Charlie Fish <contact@charlie.fish>
Reviewed-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/ContentView.swift

@@ -1116,7 +1116,7 @@ func on_open_url(state: DamusState, url: URL, result: @escaping (OpenResult?) ->
                 result(.event(ev))
             }
         case .hashtag(let ht):
-            result(.filter(.filter_hashtag([ht.string()])))
+            result(.filter(.filter_hashtag([ht.hashtag])))
         case .param, .quote:
             // doesn't really make sense here
             break