Implement MSC4142: Remove unintentional intentional mentions in replies

[tulir]

Fixes #27291
Implements matrix-org/matrix-spec-proposals#4142
Re-opened version of element-hq/matrix-react-sdk#30, which is a re-opened version of matrix-org/matrix-react-sdk#12511

The current spec is just a recommendation, so following it is not required and this can be merged already before the MSC is accepted. The goal of the MSC is to remove the dumb recommendation from the spec

For context:

• Old behavior before intentional mentions: reply fallbacks caused replies to mention the sender of the replied-to message, as well as any users explicitly mentioned in the replied-to message
• Current behavior in Element: replies mention the sender of the replied-to message, as well as all users explicitly or implicitly mentioned in the replied-to message. Implicit mentions includes any mentions from earlier in the reply chain (like the senders of all previous messages and any users ever mentioned in the chain)
• New behavior after this PR: replies mention the sender of the replied-to message

In all cases, new explicit mentions are also of course included

[florianduros]

Hi!

The MSC has not landed yet, so I'm turning back this PR to draft.
Feel free to mark as ready to review when the MSC state will change.

[tulir]

@florianduros the MSC being accepted requires product review here, so blocking this on the MSC landing would cause a deadlock 😅 matrix-org/matrix-spec-proposals#4142 (comment)

As mentioned in the PR description, the spec already allows changing the behavior in implementations, so this can be merged before the MSC lands, but it'd also work to get product approval here first, then merge the MSC, and finally merge this PR.

[richvdh]

this comment looks incorrect now

[richvdh]

LGTM, but I believe @langleyd is checking this with the product folks.

I don't quite agree with @tulir that rejecting this would cause a deadlock, because we don't have to merge changes in EW before an MSC can land; however, I do agree that this change is consistent with the current spec and therefore we don't need the MSC to land before we merge this.

In other words, the two changes can land in either order.