Override module to specify that we're overriding

What does this MR do?

Introduce a Override module to specify that we're overriding

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

CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16131

Why was this MR needed?

raise NotImplementedError unless defined?(super) doesn't work well

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #1819 (closed)