MSC4268: Sharing room keys for past messages (#4268)

* template

* wip

* Update content

* fix typos

* Introduce new 'withheld' code for unshared sessions

* Various updates

* Alice should download keys

* Apply suggestions from code review

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

* Update proposals/4268-encrypted-history-sharing.md

* Clarify new withheld code usage

* Apply suggestion from @richvdh

Fix typo

* Various clarifications

* Clarify behaviour regarding users leaving rooms

* Apply suggestions from uhoreg

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

* Add a couple more advisory notes.

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Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

Author: 3 people