Fix invite-notify and move it to an extension

Fix invite-notify and move it to an extension

The existing approach to invite-notify is deeply flawed--it currently
notifies only the target user's server, and that can't be fixed without
sending notifies for invites that end up not happening.

I'm resolving this by broadcasting a second message, INVITED, from the
target user's server. I'm also pulling it out into an extension while
I'm at it--invite notifies reveal new information, so I don't think
they should be mandatory.