added on_delete in models an migrations for django 2.0

compatibility.

src/content/migrations/0001_initial.py

@@ -1,9 +1,9 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.db import models, migrations
 import django.db.models.deletion
 from django.conf import settings
+from django.db import models, migrations
 
 
 class Migration(migrations.Migration):
@@ -44,7 +44,8 @@ class Migration(migrations.Migration):
                 ('date_modified', models.DateTimeField(
                     auto_now=True, verbose_name='Bearbeitet')),
                 ('author', models.ForeignKey(
-                    verbose_name='Autor', to=settings.AUTH_USER_MODEL)),
+                    verbose_name='Autor', to=settings.AUTH_USER_MODEL,
+                    on_delete=models.CASCADE))
             ],
             options={
                 'ordering': ('-date_created',),
@@ -144,7 +145,8 @@ class Migration(migrations.Migration):
             model_name='article',
             name='category',
             field=models.ForeignKey(
-                verbose_name='Kategorie', to='content.Category'),
+                verbose_name='Kategorie', to='content.Category',
+                on_delete=models.CASCADE),
         ),
         migrations.AlterUniqueTogether(
             name='page',