fix(ui): only show one silence per Alertmanager cluster

HA clusters share silences which means that they are currently duplicated for each alertmanager instance in that cluster

ui/src/Models/API.js

@@ -11,6 +11,7 @@ const Annotation = PropTypes.exact({
 
 const APIAlertAlertmanagerState = PropTypes.exact({
   name: PropTypes.string.isRequired,
+  cluster: PropTypes.string.isRequired,
   state: AlertState.isRequired,
   startsAt: PropTypes.string.isRequired,
   endsAt: PropTypes.string.isRequired,