When we do an invite rejection, save the signed leave event to the db

During a rejection of an invite received over federation, we ask a remote
server to make us a `leave` event, then sign it, then send that with
`send_leave`.

We were saving the *unsigned* version of the event (which has a different event
id to the signed version) to our db (and sending it to the clients), whereas
other servers in the room will have seen the *signed* version. We're not aware
of any actual problems that caused, except that it makes the database confusing
to look at and generally leaves the room in a weird state.