Use short dates when selection notifications for deletion.

This means we will retain notifications for a full week and not
delete records that are 7 x 24 hours older than the time of the run of
the deletion task.

Also the task only needs to run once a day now, so I have changed
the celery config for the deletion tasks.

config.py

@@ -1,4 +1,5 @@
 from datetime import timedelta
+from celery.schedules import crontab
 from kombu import Exchange, Queue
 import os
 
@@ -51,12 +52,12 @@ class Config(object):
         },
         'delete-failed-notifications': {
             'task': 'delete-failed-notifications',
-            'schedule': timedelta(minutes=60),
+            'schedule': crontab(minute=0, hour=0),
             'options': {'queue': 'periodic'}
         },
         'delete-successful-notifications': {
             'task': 'delete-successful-notifications',
-            'schedule': timedelta(minutes=31),
+            'schedule': crontab(minute=0, hour=0),
             'options': {'queue': 'periodic'}
         }
     }