MDL-9506 Stuck on grade_category->generate_grades. I cannot figure out how to recursi...

MDL-9506 Stuck on grade_category->generate_grades. I cannot figure out how to recursively generate raw grades for each category's associated grade_item based on that category's children categories and items. Heaps of other changes with this commit, including a new grade_object::update_from_db() method, which uses the state of the record in DB to update the current object with a matching id (useful when you insert an incomplete object in the DB and want to get the default values as set up in the DB).