Added new endpoint to get the new template stats

Added a new endpoint which combines the usage of the stats table and the
data from the notifications tables, instead of using all the data from
the notification_history table. This should speed up the query times
and improve the page performance.

- Updated to make the stats create and update function transactional as
it actually wasn't committing the data to the table
- Added the get from the stats table
- Add a a method to combine the two results
- Added the endpoint

app/celery/scheduled_tasks.py

@@ -408,7 +408,7 @@ def check_job_status():
         raise JobIncompleteError("Job(s) {} have not completed.".format(job_ids))
 
 
-@notify_celery.task(name='daily-stats-template_usage_by_month')
+@notify_celery.task(name='daily-stats-template-usage-by-month')
 @statsd(namespace="tasks")
 def daily_stats_template_usage_by_month():
     results = dao_fetch_monthly_historical_stats_by_template()