Move Fast-Forward Merge to CE

The EE counterpart https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2937

What does this MR do?

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

If you run into the following error,

NoMethodError at /h5bp/html5-boilerplate/edit
undefined method `merge_requests_ff_only_enabled' for #<Project id:8 h5bp/html5-boilerplate>

You can run the migration manually (thanks @pedroms),

rails console

require Rails.root.join("db/migrate/20150827121444_add_fast_forward_option_to_project.rb")
AddFastForwardOptionToProject.new.add

require Rails.root.join("db/migrate/20141126120926_add_merge_request_rebase_enabled_to_projects.rb")
AddMergeRequestRebaseEnabledToProjects.new.up

---

Yes, we need to make sure that we also have EE version of this MR. For example, some of the code from EE sections should be moved to CE sections.

Why was this MR needed?

Screenshots (if relevant)

Merge request widget

Conflicts	EE	CE
false
true

Project -> Merge request settings

EE	CE

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?

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