Initial handling of tombstoned/upgraded rooms

With the new behavior, it should be that:

1. Having an outdated room ID for the main room in the bot's parameters will have the bot join the successor room upon startup.
2. The bot will always join the predecessor room upon startup, if it has one.
3. Greetings will consider the predecessor room, switching to the rejoined message if the user was in it before.
4. More single syncs are done at startup, to make sure things are up-to-date when doing all of this and improve robustness on joining new rooms.

This was tested a bunch in #bots:flip.earth, so I am fairly confident it works as expected. This would solve #35 (closed).