Add username to awareness payload

What does this MR do and why?

This adds the username of the User model to the payload transmitted via the AwarenessChannel. The information is used in the frontend to show the username next to the name of the user. This was previously not considered necessary in the design proposals, but is now required.

Related frontend MR.

Follow up to !92320 (merged).

Screenshots or screen recordings

How to set up and validate locally

Covered by a spec. The frontend MR is a draft.

MR acceptance checklist

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