Add missing __init__.py for accounting migrations
Fix references to cls.teachers() in JSONDataModule
Update QSD fetching in catalog_cached query

We had a big .extra() call in ClassManager.catalog_cached() to improve
performance.  This call included a list of the IDs of QSD objects for the
class subject's index page.  I rewrote the query to use the new schema but
I am concerned about 2 things:
  1) The performance and correctness of the query (looking for
     /Classes/[emailcode]/ in the QSD's url)
  2) Whether this is necessary after the inherent performance improvement
     of schema simplification