Resolve "Prefill discussion reply textarea with usernames of other discussions participants"

What does this MR do?

Prefill discussion reply textarea with usernames of other discussions participants

Are there points in the code the reviewer needs to double check?

I do not know about it.

Why was this MR needed?

To automatically be prefilled with the usernames of everyone else (but me) who was involved in the discussion when hitting the Reply... button on an MR discussion. See: #43288 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43288 (moved)