Add Mock CI service/integration

What does this MR do?

Add MockCiService service/integration.

See doc/user/project/integrations/mock_ci.md in diff

Example mock CI server, https://gitlab.com/madlittlemods/gl-mock-ci-service https://gitlab.com/gitlab-org/gitlab-mock-ci-service

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

✅ How can we merge this in for other development situations but disable normally?

• Just comment things out? (I wish it would be easier to reactivate then because you have to know where all the pieces are)
• Some configuration 😕
• Conditonally adding things according to Rails.env.development?, thanks @DouweM

You may need to visit http://localhost:3000/admin/application_settings/services before it starts working. I was getting an error about build_mock_ci_service not being a function or nil something and seemed to just start magically working afterwards and I can't reproduce anymore.

Why was this MR needed?

To debug UI that requires a CI integration and mock the responses to get in a certain state. Ex. https://gitlab.com/gitlab-org/gitlab-ce/issues/28010

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Todo

• Add some docs for the integration
• EE branch https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1310

What are the relevant issue numbers?