Add possibility to register MUCs

[jplitza]

Some regional MUCs might want to have the warnings directly in the MUC. The idea to authorize the (un)registration would be the following:

1. Someone invites the bot to the MUC
2. The bot joins the MUC and receives the list of members, in particular the list of MUC administrators
3. An administrator sends the bot a registration command via its MUC JID ([email protected]/nina)
4. The bot stores the additional information that this is a MUC registration and hence rejoins the MUC after restart.

Optionally, one could implement a "timeout" for 3. to occur, so that the bot actively leaves the MUC if no registration occurred (vs. on restart, since it doesn't even store the MUC persistently without the registration command)