Gelencsér Szabolcs / circlestack

django.db.models.field.subclassing.SubfieldBase removed: https://docs.djangoproject.com/en/1.11/releases/1.10/#features-removed-in-1-10
New field classes require to call full_clean() method on Models, because the to_python() din't call automatically.
Related docs: https://docs.djangoproject.com/en/1.11/releases/1.8/#subfieldbase