Put connectivity down warning on the screen

[bdmendes]

The user should know when data could not be fetched from the server due to the connectivity, and that it is being loaded from the database, in a clear way. Possibly related to #515.

[thePeras]

Focusing on the connectivity task, a simple way to do it is by placing a top snack bar to show when there is no connection.

One of this packages may be useful: https://pub.dev/packages/flutter_offline https://pub.dev/packages/connectivity_plus

About the message of the content being loaded from database, I have no idea