Fix RoomView stuck in 'accept invite' state
After accepting a 3pid invite.
Rather than clear the joining flag when the join request completes,
leave it so the RoomView can see that we're expecting the user to
be joined in the various stages that might go through (waiting for
join request, waiting for room object, waiting for 'joined' member
event). The problem in this case was that we had to wait a bit for
the last one, and there was no bit of state to represent it.
This hopefully also makes the logic somewhat simpler.
Fixes https://github.com/vector-im/riot-web/issues/5041
