GYU: Implement MSC3952 in matrix-react-sdk

[Johennes]

Picking up from matrix-org/matrix-react-sdk#9983

[clokep]

There's some ongoing thought of how to deal with when you've been unmentioned via an edit: matrix-org/matrix-react-sdk#9983 (comment); I'm punting this work a bit because it isn't directly related to implement MSC3952.

[clokep]

Remaining work after matrix-org/matrix-react-sdk#9983:

• Ensure edited messages appear in red when the user was ever notified for that event (or any of its edits). (See conversation around here.)
• Better handling of editing a reply (currently the user you're replying to will be dropped from the list of mentioned users).

We might also want to file follow-ups to:

• Display something if you're "mentioned" by an event, but not in the text (no pill). (Some sort of info tooltip? This needs design help!)
• Ensure that yourself having a pill doesn't appear red unless the message is also a mention.

[daniellekirkwood]

I think we can go ahead and make some of those items of this ticket (rather than comments). Let's do that together in our next call :)

[clokep]

Ensure edited messages appear in red when the user was ever notified for that event (or any of its edits). (See conversation around here.)

Ensure that yourself having a pill doesn't appear red unless the message is also a mention.

See #24923.

Better handling of editing a reply (currently the user you're replying to will be dropped from the list of mentioned users).

See #24924.

Display something if you're "mentioned" by an event, but not in the text (no pill). (Some sort of info tooltip? This needs design help!)

See #24927.

I also filed #24925 to properly handle this in the WYSIWYG editor.