Adding migration to complete !4

ippisite/ippidb/migrations/0021_auto_20190312_1520.py

+# Generated by Django 2.0.13 on 2019-03-12 15:20
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('ippidb', '0020_contribution'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='proteindomaincomplex',
+            name='domain',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, to='ippidb.Domain'),
+        ),
+    ]