Lately we've been fetching messages only after entering the MUC, so that
we already have occupants to attach to them (due to `fetchMembers` being
called before) and thereby avoid rerenders.

I've now moved message fetching into the `initialize` method and added
missing event handlers for attaching/removing the occupant from a
message as it comes online or goes offline.

We still avoid (some) rerenders because we fetch and wait for cached
occupants before fetching cached messages and we wait for `fetchMembers`
before triggering `enteredNewRoom` which causes MAM messages to be
fetched.