Improve offline mode display

[veloce]

While in offline mode (offline status obtained with connectivityProvider):

In puzzle tab:

• remove error messages for dashboard and history that could not load

In play tab:

• disable friends button

In the bottom navigation bar:

• disable the "watch" button
• show a toast: "Not available in offline mode" if the watch button is tapped

[georgkrause]

I reviewed the issue and I think everything is solved already:

• remove error messages for dashboard and history that could not load -> Cannot reproduce, don't know where this was changed
• disable friends button -> Offline mode behavior #645
• disable watch button -> Disable Watch tab when offline #1028
• show a toast: -> Offline mode behavior #645

The only thing that might be left is a visual indicator, that the watch button is inactive in the button bar.