What does this MR do?

The components for MR create/edit and project settings share a lot of functionality. This MR updates the existing FE components for their reuse in MR create/edit.

Please note! This is part of a larger MR. You can see the full functionality here https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9168

Instructions!

• Please note that this does not point to master.
• Do not merge until the related CE MR's are merged and the corresponding commits have been rebased out.
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24435
  • https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24436

What are the relevant issue numbers?

• #1979 (closed) (FE #8350 (closed))

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• 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