Update user sort

Update the user model so it also sorts by power level and update how we initialize the model to improve performance.

The following is also changed:

• Store a single UserListModel in RoomManager and use it for everything, this means we don't create extra models (incluiding the long initialisation for each in big rooms)
• By using the single model once it has loaded the users of the new room opening and closing the draw now happens instantly (previously the model would have to be loaded every time the drawer was opened).
• To stop the initial loading and room change of Neochat slowing down (as the UserListModel would be loaded before the TimelineView is shown) the initialisation of the model is delayed until the TimelineView has loaded. This prioritises showing some messages in the timeline over populating the model so in large rooms the user list will initially be blank, but this keeps the initial load snappier.