Add allowed command

Should fix #849

There are a couple of bits that aren't great - exposing EnsureAllowed on IChatService is probably the worst, and I dunno how good the tests are (they do pass though).

Lets you type /allowed [roomname] where roomname is optional (defaults to the current room).  If you could join the room, it'll list the users who are allowed to join the room (or tell you "Anyone is allowed in general-chat as it is not private" if it's public).