I broke this in 409b6da160 making it not
possible to join a succeeding room. I decided to keep the old behavior
of not prompting you to join, because:
1. You are intentionally choosing to tap the button.
2. Room successions aren't random, a privileged user in the room is
creating it and there's already some implicit existing trust
(Also the current join confirmation dialog isn't useful enough yet to
make an actual informed decision anyway.)
466cfd971d