Create rebased ref and fast-forward to merge

• Given that the option to rebase and merge defined in Implement Rebase and Merge with One Click check... (#519379) is enabled
• When checking if a merge request can be merged, relax the requirement that ff_merge_possible? and check if the source branch can be rebased instead
• When merging (auto-merge or otherwise)
  • Create a scoped ref that points to the source branch rebased on the target branch (consider using MergeRequests::CreateRefService)
  • fast-forward merge the scoped ref into the target branch
• Human readable errors should be raised so that they can be persisted on the merge_requests.merge_error if they occur in the process
• Consider rolling out behind a feature flag