XEP-0421: Anonymous unique occupant identifiers for MUCs - Maintain author metadata for MUC messages

[jcbrand]

Currently, when a non-member leaves a MUC, we destroy the Occupant model for that user.

This will however mean that XEP-0317 Hats information (and any other presence-based metadata) will be lost.

We should instead implement support for XEP-421 and then keep Occupant instances for as long as we have corresponding messages for that user.

[jcbrand]

We now support XEP-0421. Next step would be to keep occupants for as long as they have messages in the chat history