fix(ui): pass configured alertmanager headers when making requests from the browser

Right now configured headers are only set on requests made from the backend to alertmanager API.
With this change fetch() calls in the browser will use those headers if proxy mode is not enabled.

ui/src/Models/API.js

@@ -70,6 +70,7 @@ const APIAlertmanagerUpstream = PropTypes.exact({
   cluster: PropTypes.string.isRequired,
   uri: PropTypes.string.isRequired,
   publicURI: PropTypes.string.isRequired,
+  headers: PropTypes.object.isRequired,
   error: PropTypes.string.isRequired,
   version: PropTypes.string.isRequired,
   clusterMembers: PropTypes.arrayOf(PropTypes.string).isRequired