Returns the list of past assignments the user did not submit for
            before the hard deadline.

def gone_assignments(self):
        '''
            Returns the list of past assignments the user did not submit for
            before the hard deadline.
        '''
        # Include only assignments with past hard deadline
        qs = Assignment.objects.filter(hard_deadline__lt=timezone.now())
        # Include only assignments from courses this user is registered for
        qs = qs.filter(course__in=self.user_courses())
        # Include only assignments this user has no submission for
        return qs.order_by('-hard_deadline')