Display a message if backend cannot be reached

[gastaldi]

When deploying the welcome app + app backend in OpenShift it may take a few minutes for the backend to boot. It would be nice if the welcome app could display a message saying that the backend is not available yet and fallback to a disconnected state (buttons that calls the app backend would be disabled) and move back to a connected state when the backend goes online (check every 5s or so).