Only handle self-presence for rooms we are actively managing.
Earlier we blindly react to any self-presence
stanza by calling 'ui_room_join'.
The join could have been initiated by another client or an external
Gateway. In the case of #731 this was Slack.
I remember that more people reported this kind of bug in our MUC.
We then would open a window and the nick would be (null) because we
don't have any nickname for that room set.
So only react on self-presence if we manage the MUC.
Other XMPP clients do the same.
Fix https://github.com/profanity-im/profanity/issues/731
9fb45b543b