Implement exerimental support for MSC3389

This issue has been migrated from [#12286](https://github.com/matrix-org/synapse/issues/12286).

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MSC3389 tweaks the redaction behavior or relations so that the `m.relates_to` field is not redacted.

This is particularly important for threads since redacting a message which is part of a thread causes the "Message deleted" method to jump into the main timeline (since there's no way to know that the event is part of that thread!)

This requires an experimental room version and tweaking the redaction algorithm.