MSC1767: Extensible event types & fallback in Matrix (v2) (#1767)

* first cut of MSC1767 for extensible events (replaces MSC1225)

* tabs->spaces

* fix markdown

* fix markdown

* GFM needs two spaces after ##?

* delta from msc1225

* 2021 refresh

* Refactor to be just text messaging + schema

* Update MSC numbers

* Touchups for FCP

* *ahem*

* Rewrite MSC to be understandable?

* Update proposals/1767-extensible-events.md

* Rewrite MSC, again

* Clarify Andy's blog state

* Relax m.markup's requirements

* Misc clarifications

* Revert "Relax m.markup's requirements"

This reverts commit 5f15b9f.

* Clarify push rules handling

* Fix example

* Update extensible events MSCs list

* Update Andy's blog post reference

* Update per review feedback

* Move emotes, notices, and encryption out to dedicated MSCs
* Clarify text throughout

* Cover all the bases

* Clarify mixins are exactly what they're assumed to be

* Clarify fallback and number of datums per event

* Fix description of plain text baseline

* Clarify SCT's role in feature scope & cut "mixed" events (not mixins)

* Fix description of mixins

* Update proposals/1767-extensible-events.md

Co-authored-by: David Baker <dbkr@users.noreply.github.com>

* Expand on how unknown events are handled

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Trim line length post-suggestions

* Mention that extensible events might appear in other room versions early

* Remove duplicated unknown parse order being an implementation detail

* Note difference between optional content blocks and mixins

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Update proposals/1767-extensible-events.md

Co-authored-by: Alexey Rusakov <Kitsune-Ral@users.sf.net>

* Clarify that clients are still strongly encouraged to validate HTML

* Fix mention of mixing legacy event types

* Update proposals/1767-extensible-events.md

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Link to MSC3765

* Rename `m.markup` to `m.text`

---------

Co-authored-by: Travis Ralston <travpc@gmail.com>
Co-authored-by: Travis Ralston <travisr@matrix.org>
Co-authored-by: David Baker <dbkr@users.noreply.github.com>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Alexey Rusakov <Kitsune-Ral@users.sf.net>
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>