MSC2676: Message editing (#2676)

* initial version of message editing proposal

* fix MSC numbers

* Fix JSON in example

Co-authored-by: Alexandre Morignot <erdnaxeli@gmail.com>

* clarifications

* remove obsolete "XXX:", and fix a typo

* Initial cleanup and restructuring

* Clarify algorithm for replacing content

* background

* More clarifications on applying edits

* Clarify behaviour of redactions

* Minor grammar fixes

* Move the section on `msgtype` down

It clutters up the initial description - let's move it down into a more
detailed section.

* Clarify how edits are ordered

- we use event_id as a tiebreaker.

We also have a section in "Future considerations" about this, so I don't think
we need the braindump in "Edge cases".

* Spec the behaviour for encrypted events

* Requirements for an edit event to be considered valid

* Collect "client behaviour" and "sever behaviour" together

... and clarify these sections.

* Clarify permalinks section

* Notes on edits of replies

* Clarify that `m.relates_to` within `m.new_content` is ignored

* Clarifications from review

* event ids are sorted lexicographically

* Clarify aggregation section

* minor clarifications

* Clarify which endpoints support edits

* move definition of latest edit

* Apply suggestions from code review

Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

* fix typo

* Attempt to clarify encrypted events

Co-authored-by: Alexandre Morignot <erdnaxeli@gmail.com>
Co-authored-by: Richard van der Hoff <richard@matrix.org>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>