Show template usage broken down by month

This follows on from:
- https://github.com/alphagov/notifications-admin/pull/1094
- https://github.com/alphagov/notifications-admin/pull/1109

It depends on:
- [ ] https://github.com/alphagov/notifications-api/pull/829

A year is too long. Month-by-month is a better timeframe for making
decisions or seeing patterns in your usage.

app/notify_client/template_statistics_api_client.py

@@ -20,6 +20,12 @@ class TemplateStatisticsApiClient(NotifyAdminAPIClient):
             params=params
         )['data']
 
+    def get_monthly_template_statistics_for_service(self, service_id, year):
+
+        return self.get(
+            url='/service/{}/notifications/templates/monthly?year={}'.format(service_id, year)
+        )['data']
+
     def get_template_statistics_for_template(self, service_id, template_id):
 
         return self.get(