* 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>