Add support to edit a relation onto an existing event

[MadLittleMods]

Use case

When you forget to "reply" to a specific message it would be useful to edit the message and attach the reply.

See element-hq/element-meta#1047 for ideas/interest in this feature in Element.

Proposal

Nothing needs to change spec-wise except for the language to allow it (as far as I know).

This already technically works in Synapse/Element after matrix-org/matrix-react-sdk#6817

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Related to retroactive threading

Prior art

MSC2676 already explicitly calls out that this is not allowed:

• https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/2676-message-editing.md#applying-mnew_content
• https://github.com/matrix-org/matrix-spec-proposals/blob/main/proposals/2676-message-editing.md#edits-of-replies

There was some discussion on the MSC to allow for this but it was punted to a future proposal

[clokep]

This already technically works in Synapse/Element after matrix-org/matrix-react-sdk#6817

You're very much not supposed to be able to edit relations and I'm fairly sure that Synapse does not take them into account. In particular, does the /relations API or the aggregation include the new relation afterward? I suspect this is a client bug of some sort where it doesn't properly use the original m.relates_to property of the event.

[t3chguy]

https://spec.matrix.org/v1.6/client-server-api/#applying-mnew_content

Any m.relates_to property within m.new_content is ignored.

[t3chguy]

matrix-org/matrix-js-sdk#3178 aligns Element Web with Spec & Synapse