Add monthly breakdown of usage

Basically:
- shows all the months from start of given financial year to now or end
  of given financial year (whichever is earliest)
- shows a breakdown of free and paid text messages for each of these
  months

Depends on:
- [x] https://github.com/alphagov/notifications-api/pull/699

app/notify_client/service_api_client.py

@@ -211,6 +211,9 @@ class ServiceAPIClient(NotificationsAPIClient):
     def update_whitelist(self, service_id, data):
         return self.put(url='/service/{}/whitelist'.format(service_id), data=data)
 
+    def get_billable_units(self, service_id, year):
+        return self.get(url='/service/{}/billable-units?year={}'.format(service_id, year))
+
 
 class ServicesBrowsableItem(BrowsableItem):
     @property