Added support for internal notes on merge requests

What does this MR do and why?

Updates the comment form on merge requests to support internal notes. Internal notes can only be normal notes or discussions, not diff discussions.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

• Visit a merge request
• The Make this an internal note checkbox should be visible

#362834 (closed)