WIP: Move fast-forward merge request code around for CE port

What does this MR do?

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14272

• Move fast-forward merge request code around for CE port
  • "Fast-forward merge" setting at /namespace/repo/edit
  • Fast-forward normal state and conflict state for MR widget

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

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

What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/20076