Fix modchat in unofficial rooms

For canTalk, properly inherit user group from official rooms, in agreement with commit a153f2f51e2139f56b2ca11ed3d0c1c504be4fef

This fixes issues with modchat of level & and ~ in unofficial chat rooms: absolutely nobody was able to talk if they were set, and they could not be turned off.

Also, modchat % and + will now be removable by global moderators.