Branch unprotection restrictions disable UI

What

Disables the update/unprotect UI when a protected branch has restrictions added to control which users can do so.

Why

Visually inform users that they are unable to perform an action. Previously clicking "Unprotect" sent the user to a blank page and update attempts presumably resulted in a javascript error.

These restrictions are currently set through the API and this MR updates the UI to reflect that.

Screenshots

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
• Conform by the merge request performance guides
• End-to-end tests pass (package-qa manual pipeline job)

Related

• Second part of https://gitlab.com/gitlab-org/gitlab-ee/issues/4800