* Consistently use room_id from federation request body
Some federation APIs have a redundant `room_id` path param (see
https://github.com/matrix-org/matrix-doc/issues/2330). We should make sure we
consistently use either the path param or the body param, and the body param is
easier.
* Kill off some references to "context"
Once upon a time, "rooms" were known as "contexts". I think this kills of the
last references to "contexts".
