Add connectivity lost warning message

If the app loses connection to the backend server, it should trigger a warning message to the user, regardless the screen he is in. This service must be general and consistent across the whole application, and provide almost immediate feedback connectivity has been lost.

Toast notifications can be used for this. It’ll be notified to the user immediately if connection to the server is lost. This can work either when trying to connect (handling an error) or with a periodic ping of every 5 secs if the application is idle.