Add offline status page / startup progress

[jazdw]

Instead of displaying an error when the UI module cannot contact the backend / REST API we should display a user friendly message (translatable) that tells the user that the Mango backend is not available. This can be a different message if the user's internet is disconnected.

We should also display startup progress if Mango is starting up.

Basically implement a better offline experience that is more robust. This will become more important as we remove the legacy UI.

[jazdw]

Partially implemented, the app will now load when Mango is offline. The startup progress is displayed in a toast at the bottom of the screen.

[jazdw]

Especially with session persistence configured we need to popup a message/button telling the user that there is a UI update available.