Login cleanup

Lessen impact of !255 (closed) and remove minor bug

• Localization
  • lib/pages/splash_page.dart: SnackBars have to be localized.
• Styling
  • lib/common/widgets/logout_dialog.dart: Used in SplashPage and ProfilePage. Maybe later
  • lib/pages/login/greeter_page.dart: TextTheme of Hero-Widget has to be same as in lib/pages/login/widgets/login_header.dart.