Do not show the 'debug' banner

It's annoying, and does not allow us to easily take screenshots.
Specially since the apps built in release mode cannot be run in an x86
android emulator.

lib/app.dart

@@ -44,6 +44,7 @@ class JournalApp extends StatelessWidget {
         '/settings': (context) => SettingsScreen(),
         '/setupRemoteGit': (context) => OnBoardingScreen(onCompleted),
       },
+      debugShowCheckedModeBanner: false,
     );
   }
 }