In #20135 we prevented invalid inputs from being accepted in category setting form fields on the front-end. We didn't do anything on the back-end at that time, because we were still discussing which path we wanted to take. Eventually we decided we want to move this to a new CategorySetting model.
This PR moves the require_topic_approval and require_reply_approval from custom fields to the new CategorySetting model.
This PR is nearly identical to #20580, which migrated num_auto_bump_daily, but since these are slightly more sensitive, they are moved after the previous one is verified.
