Federated Invites: It appears that there is no way to retract an invite such that the invitee is made aware of it

Nobody has claimed this yet.

If you invite a user over federation and then change your mind (retract the invite, by setting their `m.room.member` membership to `leave`), there doesn't seem to be any mechanism to notify the invitee's server of that.

The result is that the invitee is left with a stale invite and when they try to redeem it, they get some generic error like 'Failed to make_join via any server' (Synapse example).

This is probably limited to the case where there aren't other users in the room on the same server as the invitee, otherwise the `m.room.member` event would be sent to the invitee's homeserver (*by coincidence*) and that probably leads to the right thing happening.

It seems there should be:

* a Federation endpoint to notify about a retracted invite, but since that can't be guaranteed to be reliable, we also need...
* an error code on `make_join` and `send_join` somehow informing the invitee that their invite has since been retracted, so clients can show an appropriate error message

Very similar in spirit to #146

Start with the federation invite flow described in the issue, especially retracting an invite by setting m.room.member membership to leave. Review the make_join and send_join entry points and the related discussion in #146. Done means the specification defines how retracted invites are communicated and how invitees receive an appropriate error.

Written by the indexing model from the issue text.