Add a 'Ping' endpoint to test authorization status

Currently the Admin UI will send a GET request to "ag-push/rest/applications"
endpoint on each page transition to make sure the the user is authenticated before loading the page.

This was a quick and dirty way of doing it and it is sort of missing using that endpoint.

I would like to add a "Ping" endpoint, that is @Secured and only returns a 200

Something like this:

@Stateless
@Path("/ping")
@Secure({ "developer", "admin" })
public class Ping {

    @GET
    public Response ping() {
        return Response.ok().build();
    }
}