confirmation: Add on_delete in foreign keys.

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

confirmation/migrations/0001_initial.py

@@ -2,6 +2,7 @@
 from __future__ import unicode_literals
 
 from django.db import models, migrations
+import django.db.models.deletion
 
 
 class Migration(migrations.Migration):
@@ -18,7 +19,7 @@ class Migration(migrations.Migration):
                 ('object_id', models.PositiveIntegerField()),
                 ('date_sent', models.DateTimeField(verbose_name='sent')),
                 ('confirmation_key', models.CharField(max_length=40, verbose_name='activation key')),
-                ('content_type', models.ForeignKey(to='contenttypes.ContentType')),
+                ('content_type', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='contenttypes.ContentType')),
             ],
             options={
                 'verbose_name': 'confirmation email',