Make created_at a property of contact lists

The view layer shouldn’t be having to deal with converting dates as
strings. This is an artefact of how we send data from the API to the
admin app. The model layer should be responsible for turning JSON into
richer types, where it can.

tests/app/models/test_contact_list.py

@@ -0,0 +1,9 @@
+from datetime import datetime
+
+from app.models.contact_list import ContactList
+
+
+def test_created_at():
+    created_at = ContactList({'created_at': '2016-05-06T07:08:09.061258'}).created_at
+    assert isinstance(created_at, datetime)
+    assert created_at.isoformat() == '2016-05-06T08:08:09.061258+01:00'