Autofocus the editor when commenting on a diff

What does this MR do and why?

Autofocus the rich text editor when commenting on a diff

Changelog: fixed

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
No autofocus Screen Recording 2023-07-19 at 18.23.04.mov	Autofocus Screen Recording 2023-07-19 at 18.22.17.mov

How to set up and validate locally

1. Make sure rich text editor is the default. Ensure this by clicking Switch to rich text editing in a text box in issues.
2. Go to an MR.
3. Add a comment on a diff line in the MR.
4. Notice that the rich text editor receives autofocus.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #419090 (closed)