Use Enum and constraint for valid notification types.

fedireads/migrations/0016_auto_20200313_1337.py

@@ -0,0 +1,22 @@
+# Generated by Django 3.0.3 on 2020-03-13 13:37
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('fedireads', '0015_auto_20200311_1212'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='notification',
+            name='notification_type',
+            field=models.CharField(choices=[('FAVORITE', 'Favorite'), ('REPLY', 'Reply'), ('TAG', 'Tag'), ('FOLLOW', 'Follow'), ('FOLLOW_REQUEST', 'Follow Request')], max_length=255),
+        ),
+        migrations.AddConstraint(
+            model_name='notification',
+            constraint=models.CheckConstraint(check=models.Q(notification_type__in=['FAVORITE', 'REPLY', 'TAG', 'FOLLOW', 'FOLLOW_REQUEST']), name='notification_type_valid'),
+        ),
+    ]