Allow tagging messages.

[RokeJulianLockhart]

Your use case

What would you like to do?

I want to tag messages according to custom tags I can create, like I can for e-mail messages via https://mail.proton.me/ (I specify this because https://outlook.live.com/ and https://mail.google.com/mail/ solely provide this per conversation, rather than per message).

Why would you like to do it?

To triage messages, based upon whether they are taskless, incomplete, or complete, and their importance.

How would you like to achieve it?

I want Element to implement this functionality.

Have you considered any alternatives?

#2404 (comment)

Additional context

matrix-org/matrix-spec#594 (comment) seems like it might be relevant, but what it describes isn't obvious to me. #347 (comment) demonstrates a use case.