Edit Branch Rules: Add Branch protection: Allowed to merge

See &12524 (closed) for details

Allowed to merge

• Allows editing of roles, users, and groups
• Default:
  • Roles: Administrators, Maintainers
• Empty state text: No one is allowed to merge changes.
• On change, display toast "Allowed to merge updated"
  • Undo action would be nice to have.

Default	Edit - default	Edit - filled	Allowed to merge updated