This adds support for the stable identifiers of MSC2285 while
continuing to support the unstable identifiers behind the configuration
flag. These will be removed in a future version.
* Changes hidden read receipts to be a separate receipt type
(instead of a field on `m.read`).
* Updates the `/receipts` endpoint to accept `m.fully_read`.
This makes the serialization of events synchronous (and it no
longer access the database), but we must manually calculate and
provide the bundled aggregations.
Overall this should cause no change in behavior, but is prep work
for other improvements.
