Fixed multiple problems related to migration to django-preferences.

app/YtManagerApp/migrations/0008_auto_20181229_2035.py

@@ -0,0 +1,25 @@
+# Generated by Django 2.1.2 on 2018-12-29 20:35
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('YtManagerApp', '0007_auto_20181029_1638'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='usersettings',
+            name='user',
+        ),
+        migrations.RenameField(
+            model_name='subscription',
+            old_name='delete_after_watched',
+            new_name='automatically_delete_watched',
+        ),
+        migrations.DeleteModel(
+            name='UserSettings',
+        ),
+    ]