remove RemovedInDjango20Warning:
on_delete will be a required arg for ForeignKey in Django 2.0.
Set it to models.CASCADE on models and in existing migrations if you want to
maintain the current default behavior.
See https://docs.djangoproject.com/en/1.11/ref/models/
fields/#django.db.models.ForeignKey.on_delete
Valentin Deniaud
Lauréline Guérin
Frédéric Péters