Clean up old failed uploads

1)

Make a model manager hiding

Data.upload_completed_successfully = False

Something like how we manage ChaHub users:

class ChaHubUserManager(UserManager):
    def get_queryset(self):
        return super().get_queryset().filter(deleted=False)

    def all_objects(self):
        return super().get_queryset()

2)

Clean up uploads that failed > 3 days ago:

older_than_3_days = now()-timedelta(days=3)

# may need to access model manager differently, like Data.objects.include_unsuccessful ...

Data.objects.filter(upload_completed_successfully=False, created_when__gte=older_than_3_days).delete()