-- Changed docstrings to use double quote.
-- More consistent naming for youtube IDs (normal_speed_id -> youtube_id_1_0).
-- Formatting changes for readability.
-- Updated test XML to expect JSON-encoded strings; prevents tests from spuriously failing.

Update XML in old video tests. This change tags like <video youtube="1.0:abcdefg"> to <video youtube_id_1_0="abcdefg".

Update XML in old video tests. This change tags like <video youtube="1.0:abcdefg"> to <video youtube_id_1_0="abcdefg".

* 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.