Resolve HTML/CSS inconsistent comment forms on Issue and Merge Request

What does this MR do?

Fixes HTML/CSS inconsistent new comment forms on Issue and Merge Request.

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

Migration of the CSS selectors should be checked by reviewers.

Why was this MR needed?

Comment forms on Merge Request are too narrow unlike ones on Issues due to !14061 (merged).

Screenshots (if relevant)

TBD

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #44870 (closed)