Manage MessageContentModels properly so we don't leak memory.

- Manage MessageContentModels properly so we don't leak memory creating new ones every time the role is refreshed.
- Parent and reply MessageContentModels to their message to make sure they get cleaned up when the parent is deleted.
- Make sure ReactionModels are cleaned up on room change to stop that list just growing.

src/neochatroom.cpp

@@ -1748,6 +1748,7 @@ void NeoChatRoom::downloadEventFromServer(const QString &eventId)
     connect(job, &BaseJob::success, this, [this, job, eventId] {
         // The event may have arrived in the meantime so check it's not in the timeline.
         if (findInTimeline(eventId) != historyEdge()) {
+            Q_EMIT extraEventLoaded(eventId);
             return;
         }