Fix some races in the chatroom joining code

Fix some races in the chatroom joining code

The auto-reconnection logic for chatrooms was a bit odd. It would for
every chatroom that existed when the account manager was prepared
connect once to status-changed for each account for each chatroom...

Change this such that whenever an account connects, we simply
get the then current list of chatrooms and auto-connect those as needed.

This fixes issues when a chatroom gets removed from the manager and when
the auto-connect options on a chatrooms change.