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 num_auto_bump_daily from custom fields to the new CategorySetting model.
In addition it sets the default value to 0, which exhibits the same behaviour as when the value is NULL.