models: add migration to on_delete=protect

swh/deposit/migrations/0017_auto_20190925_0906.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.23 on 2019-09-25 09:06
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('deposit', '0016_auto_20190507_1408'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='deposit',
+            name='parent',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.PROTECT, to='deposit.Deposit'),
+        ),
+    ]