This takes Cale's suggestion and uses a separate table for individual
student overrides, rather than overloading StudentModule.state.  Since
there is a new table, this commit will require running 'migrate'.

I've also gone ahead and configured common.py for everything to be
disabled by default.  To use IDDE and the student overrides feature, add
the following to your private.py:

    FEATURES['INDIVIDUAL_DUE_DATES'] = True
    FIELD_OVERRIDE_PROVIDERS += ('courseware.student_field_overrides.IndividualStudentOverrideProvider',)