Create system note for attention requests

What does this MR do and why?

This MR creates a system note when a reviewer or assignee's attention is requested or no longer requested, as wanted in #345367 (closed).

Screenshots or screen recordings

How to set up and validate locally

1. Enable the mr_attention_requests feature

   Feature.enable(:mr_attention_requests)

2. Visit an open MR, such as http://localhost:3000/h5bp/html5-boilerplate/-/merge_requests/1
3. Add an Assignee or Reviewer
4. Click the star icon next to the Assignee or Reviewers name
5. Wait for the page to update the system notes
6. Look at the bottom of the page for the newest system note, which should be the "requested attention" one.
7. Click the star icon next to the Assignee or Reviewers name
8. Wait for the page to update the system notes
9. Look at the bottom of the page for the newest system note, which should be the "attention request removed" one.

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 #345367 (closed)