The root node of a thread (and events related to it) are considered
"part of a thread" when validating receipts. This allows clients which
show the root node in both the main timeline and the threaded timeline
to easily send receipts in either.
Note that threaded notifications are not created for these events, these
events created notifications on the main timeline.
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`.
