Looking at the Prosody code, it appears to be cheap since
Prosody will do nothing if you're already registered and this works
around a bug in mod_muc_rai where events aren't fired for each resource
of the same joined nick joining or leaving the MUC.
When a MUC starts out hidden, we first need to join in order to find out
whether we're affiliated before we can know whether we should enable RAI
or join the MUC as usual.
- Add test for incoming RAI message
- Only enable RAI if the user is affilated in MUC being left
- Handle error presence indicating a resouce-constraint
- Don't unregister stanza handlers in `leave`, since we still want to
listen to RAI-related stanzas. Instead unregister upon the `destroy`
event.
