Allow correspondents to be deleted without deleting their documents

Fixes #235

src/documents/migrations/0018_auto_20170715_1712.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-07-15 17:12
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '0017_auto_20170512_0507'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='document',
+            name='correspondent',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='documents', to='documents.Correspondent'),
+        ),
+    ]