Add UI for Health Endpoint

We have a REST endpoint, protected by Keycloak, to give feedback of the UPS health:
https://aerogear.org/docs/specs/aerogear-unifiedpush-rest/index.html#-2006359689

It returns details if the DB is present/reachable:

• https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/jaxrs/src/main/java/org/jboss/aerogear/unifiedpush/rest/util/HealthCheck.java#L59

As well as if the different supported Push Networks are working:

• https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/push/sender/src/main/java/org/jboss/aerogear/unifiedpush/message/util/HealthNetworkServiceImpl.java#L51-L56

Currently we don't really make usage of this... perhaps we should add a STATUS menu entry, see screenshot