Queryable student module app

Makes new tables that are queryable to get aggregate data on student per
problem information and student grades at three levels: overall course,
assignment type, and assignment.

Provides UI in instructor dashboard to view student metrics using
class_dashboard app.

Remove primary keys from queryable tables
    De-normalize user info of id, username and name into queryable tables.

Make all tests pass

* use default parameter	in get_or_create for non-unique-together
  values
* properly set module_state_key

Conflicts:
	lms/envs/test.py