Upgrade to Django 2.1 using pip-tools

Closes #88

apps/files/migrations/0005_auto_20180922_1717.py

@@ -18,10 +18,4 @@ class Migration(migrations.Migration):
             field=models.CharField(blank=True, default='', max_length=240, verbose_name='Image caption'),
             preserve_default=False,
         ),
-        migrations.AlterField(
-            model_name='imagefile',
-            name='credit',
-            field=models.CharField(blank=True, default='', max_length=240, verbose_name='Image credit'),
-            preserve_default=False,
-        ),
     ]

apps/files/migrations/0006_auto_20180928_1323.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.0.8 on 2018-09-28 13:23
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('files', '0005_auto_20180922_1717'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='imagefile',
+            name='credit',
+            field=models.CharField(blank=True, default='', max_length=240, verbose_name='Image credit'),
+            preserve_default=False,
+        ),
+    ]

apps/files/models.py

@@ -10,7 +10,7 @@ class BaseFile(models.Model):
         upload_to='.',
     )
     user = models.ForeignKey(
-        User, related_name='%(class)s'
+        User, related_name='%(class)s', on_delete=models.PROTECT,
     )
 
     class Meta: