disable 'leave before join'

leaving a MUC before joining it was a work around for servers that did not treat a
<x/> join as a full join and didn’t send the full user list if they thought the user was
still in the room.
this happens if Conversations restarts after an inproper disconnect. The MUC will think
the user is still in the room.

however nowadays most modern servers will treat <x/> joins as full joins. on the user hand
leave before join would trigger flood prevention on ejabberds and race the first message
with the actual join (making the message arrive before the user is considered in the room)