WIP: Add Gitlab/ModelsInheritApplicationRecordCop

What does this MR do?

The Rails 4->5 upgrade requires that existing/new models inherit from ApplicationRecord instead of ActiveRecord::Base

This MR introduces a new cop that will catch newly introduced models that inherit the old ActiveRecord::Base

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/53976

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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.