Move saving of processing time into separate task

We current do this as part of send-daily-performance-platform-stats but
now this moves it into its own separate task. This is for two reasons
- we will shortly get rid of the send-daily-performance-platform-stats
  task as we no longer will need to send anything to performance
  platform
- even if we did decide to keep the task
  send-daily-performance-platform-stats and remove the specific bits
  that relate to the performance platform, it's probably nicer to
  rewrite the new task from scratch to make sure it's all clear and easy
  to understand

app/config.py

@@ -278,6 +278,11 @@ class Config(object):
             'schedule': crontab(hour=2, minute=0),
             'options': {'queue': QueueNames.PERIODIC}
         },
+        'save-daily-notification-processing-time': {
+            'task': 'save-daily-notification-processing-time',
+            'schedule': crontab(hour=2, minute=0),
+            'options': {'queue': QueueNames.PERIODIC}
+        },
         'remove_sms_email_jobs': {
             'task': 'remove_sms_email_jobs',
             'schedule': crontab(hour=4, minute=0),