Resolve "Add admin note to user show page"

What does this MR do?

This MR adds an "Admin Note" display box on the admin user view, if an admin note has been added to the user record.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Administrators can add an "Admin Note" to a user record, but when viewing that user in the admin view you do not see that a note exists, nor do you see the content of it. You can only view it by editing the user or by hovering over the note icon next to the username in a user list.

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #1090 (closed)