Fixes #2840 - Use both the room list room and the room preview details to populate the join room screen

- the room summary API is indeed enabled on matrix.org and working fine for most rooms
- it is not however capable of giving us data about non-joined + private rooms
- the SDK addresses that by first trying to use known rooms before resorting to the preview endpoint
- that fails if it's a brand new room that the client doesn't know about yet i.e. a sync hasn't ran, which is exactly what's happening here
- the ClientProxy instead does wait for the room list to go into the first loaded before returning the room

ElementX/Sources/Screens/JoinRoomScreen/JoinRoomScreenModels.swift

@@ -29,11 +29,19 @@ enum JoinRoomScreenInteractionMode {
     case knock
 }
 
+struct JoinRoomScreenRoomDetails {
+    let name: String?
+    let topic: String?
+    let canonicalAlias: String?
+    let avatar: RoomAvatar
+    let memberCount: UInt
+}
+
 struct JoinRoomScreenViewState: BindableState {
     // Maybe use room summary details or similar here??
     let roomID: String
     
-    var roomDetails: RoomPreviewDetails?
+    var roomDetails: JoinRoomScreenRoomDetails?
     
     var mode: JoinRoomScreenInteractionMode = .loading
     
@@ -52,7 +60,7 @@ struct JoinRoomScreenViewState: BindableState {
     }
     
     var avatar: RoomAvatar {
-        .room(id: roomID, name: title, avatarURL: roomDetails?.avatarURL)
+        roomDetails?.avatar ?? .room(id: roomID, name: title, avatarURL: nil)
     }
 }