Get notification count from api instead of list of notifications

when checking if job can be cancelled. This makes query lighter,
less likely to fall over if job is large and removes
pagination bug.

app/notify_client/notification_api_client.py

@@ -127,5 +127,8 @@ class NotificationApiClient(NotifyAdminAPIClient):
             }
         )
 
+    def get_notification_count_for_job_id(self, *, service_id, job_id):
+        return self.get(url='/service/{}/job/{}/notification_count'.format(service_id, job_id))["count"]
+
 
 notification_api_client = NotificationApiClient()