MDL-9506 Category and Item now set their own courseid if not set explicitly, and...

MDL-9506 Category and Item now set their own courseid if not set explicitly, and if they have enough information from
other sources. category::set_as_parent has a new constraint: cannot set over children from different courses.
Applied recursion to grade_tree::renumber(), but still some fine-tuning to do, since only grade_items are getting updated in DB.