Removing problem from contest does not remove contest_object from affected subs

[kiritofeng]

Which in turn, corrupts the DB state.

[Ninjaclasher]

Something like adding a signal

@receiver(post_delete, sender=ContestProblem)
def contest_problem_delete(sender, instance, **kwargs):
    Submission.objects.filter(contest_object=instance.contest, contest__isnull=True).update(contest_object=None)

should fix it, but I haven't tested it.