Allow comments on the full diff file

What does this MR do and why?

This change allows users to comment on the full diff file rather than the comment having to be tied to a specific line inside of a diff.

#17120 (closed)

Screenshots or screen recordings

Screen_Recording_2023-05-30_at_10.14.41

How to set up and validate locally

• Enable comment_on_files feature flag
• Navigate to the changes tab in a merge request
• Click the action dropdown in the diff file header
• Click Comment on this file

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.