'Returns True if the user is enrolled in the course (the entry must exist
and it must have `is_active=True`). Otherwise, returns False.
`user` is a Django User object. If it hasn\'t been saved yet (no `.id`
attribute), this method will automatically save it before
adding an enrollment for it.
`course_id` is our usual course_id string (e.g. "edX/Test101/2013_Fall)'

@classmethod
def is_enrolled(cls, user, course_key):

    enrollment_state = cls._get_enrollment_state(user, course_key)
    return (enrollment_state.is_active or False)
