(Part 8) FE multiple approval rules - single rule

What does this MR do?

This MR adds support for the GitLab Starter version which only allows one approval rule. It is part of the larger !9001 (merged). To fully support this feature, there were some BE changes that needed to be made.

Screenshots

The following screenshots are under the GitLab starter license (only one approval rule allowed):

Project settings with no members selected	MR Widget with code owner rule	MR edit

Instructions

Do not merge until:

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9491 has been merged
• The BE commits on this MR have been rebased out.

What are the relevant issue numbers?

Part of #1979 (closed)

Part of #8350 (closed) (FE)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

/cc @lulalala @reprazent