Add usage to the dashboard

Takes the number of emails and SMS fragments sent from:
https://github.com/alphagov/notifications-api/pull/273

Using these numbers it’s possible to show:
- how much of your allowance is left
- or how much you have spent

For now the allowance and rates are hard coded.

Only for users that have manage service.

app/notify_client/api_client.py

@@ -159,6 +159,9 @@ class ServiceAPIClient(NotificationsAPIClient):
     def get_service_history(self, service_id):
         return self.get('/service/{0}/history'.format(service_id))
 
+    def get_service_usage(self, service_id):
+        return self.get('/service/{0}/fragment/aggregate_statistics'.format(service_id))
+
 
 class ServicesBrowsableItem(BrowsableItem):