Add a RuboCop to prevent new not_owned endpoints

Description of the proposal

I noticed that we still sometimes added these, so I figured a RuboCop was the easiest way to prevent that in the future.

This also adds a feature category for the WorkItemsController, which is the one that made me notice.

All other offences have been disabled and are really un-owned, as far as I can tell.

Check-list

• Make sure this MR enables a static analysis check rule for new usage but ignores current offenses.
• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend).
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus).
• Follow the review process as usual.
• Once approved and merged by a maintainer, mention it again:
  • In the relevant Slack channels (e.g. #development, #backend, #frontend).
  • (Optional depending on the impact of the change) In the Engineering Week in Review.

/cc @gitlab-org/maintainers/rails-backend