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.

app/models/contact_list.py

@@ -4,6 +4,7 @@ from os import path
 from flask import abort, current_app
 from notifications_utils.formatters import strip_whitespace
 from notifications_utils.recipients import RecipientCSV
+from notifications_utils.timezones import utc_string_to_aware_gmt_datetime
 from werkzeug.utils import cached_property
 
 from app.models import JSONModel, ModelList
@@ -21,7 +22,6 @@ class ContactList(JSONModel):
 
     ALLOWED_PROPERTIES = {
         'id',
-        'created_at',
         'created_by',
         'service_id',
         'original_file_name',
@@ -112,6 +112,10 @@ class ContactList(JSONModel):
             contact_list_id=self.id,
         )
 
+    @property
+    def created_at(self):
+        return utc_string_to_aware_gmt_datetime(self._dict['created_at'])
+
     @property
     def contents(self):
         return self.download(self.service_id, self.id)

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'