* apply policy per-course, even if multiple courses share course and org fields
* keep descriptors separate in xml store, so that if two such courses
change the same module in different ways, it works. Such edits will need to
merged on CMS import...
* add get_instance(course_id, location) method to replace get_item(location).
Update all the call sites
* tests, including a 2nd toy course with same course and org.
