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/models.py

@@ -1613,3 +1613,11 @@ class StatsTemplateUsageByMonth(db.Model):
         nullable=False,
         default=0
     )
+
+    def serialize(self):
+        return {
+            'template_id': str(self.template_id),
+            'month': self.month,
+            'year': self.year,
+            'count': self.count
+        }