fix contact list bst bug

the api returns UTC timestamps, we should keep them as UTC timestamps
until the very last moment, and only convert them into BST when we know
we want to return to a user (ie: in contact-list.html and other places
like that)

app/models/contact_list.py

@@ -4,7 +4,6 @@ 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
@@ -23,6 +22,7 @@ class ContactList(JSONModel):
 
     ALLOWED_PROPERTIES = {
         'id',
+        'created_at',
         'created_by',
         'has_jobs',
         'recent_job_count',
@@ -115,10 +115,6 @@ 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)