Visually show reactions while the server is processing them

Unlike messages, reactions do not have a "pending" state. This problem is more obvious while the server is under heavy load, or your connection is disconnecting often, etc. This creates a pretty terrible UX as you try to add an emoji, and nothing obvious happens.

libQuotient gives us the tools to do this, we can take advantage of this. The main missing component is that we depend on a changed RoomMessage event for reaction updates, but in the pending queue the library gives us a ReactionEvent directly. We can process this, and tell ReactionModel to use this while waiting for the message to be updated.

I did some code cleanup in ReactionModel while I'm touching it as well.