* Spec MSC3771: Threaded read receipts
Note: this builds on a (as of writing) non-existent "threading" section, which is part of a different commit.
* Spec MSC3773: Threaded notifications
* changelog
* Various clarifications per review
* Convert `m.receipt.yaml` to traditional YAML
* Spec MSC2285 (private read receipts)
* Add some obvious copyright headers
* Add changelog entries
* Appease the linter
Apparently it hates it when you do this.
* Allow m.fully_read on /receipts
* Apply suggestions from code review
Co-authored-by: Matthew Hodgson <matthew@matrix.org>
Co-authored-by: Matthew Hodgson <matthew@matrix.org>