Implement MSC4142: Remove unintentional intentional mentions in replies

[tulir]

Fixes element-hq/element-web#27291 (I think)

[tulir]

The MSC requires an opinion from product-centered folks, so marking this as ready for review. 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