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.
ea18152fad