Add ARIA regions to comments and comment form

What does this MR do and why?

Adds ARIA regions to the comment form, and the comments area, on the merge request Overview tab.

Related to gitlab-ui#1553 (moved) but does not wholly fix the problems outlined there.

Screenshots or screen recordings

Before	After

Click to see regions outlined on a MR page

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.