Category Moderators feature, phase 1; back-end only

https://meta.discourse.org/t/category-specific-moderators-phase-1-rfc/21900

- adds a moderator flag to the `CATEGORY_USERS` table
- all the checks are in `Guardian` and subclasses
- tests are mostly in `category_user_spec.rb`
- no alterations to UI or existing functionality; feature is invisible if not used