Allow correspondents to be deleted without deleting their documents

Fixes #235

src/documents/tests/factories.py

@@ -0,0 +1,17 @@
+import factory
+
+from ..models import Document, Correspondent
+
+
+class CorrespondentFactory(factory.DjangoModelFactory):
+
+    class Meta:
+        model = Correspondent
+
+    name = factory.Faker("name")
+
+
+class DocumentFactory(factory.DjangoModelFactory):
+
+    class Meta:
+        model = Document