Attempt at implementing https://dev.louiz.org/issues/3401
Adds a command that invites people into a newly created room (with a randomly generated name), on a configured muc component, or the one available on the user's domain.
Update:
I implemented a /impromptu command, usable everywhere in poezio, that takes JIDs as argument and create a new room.
I also changed /invite so that it can be used in 1:1 tabs.