migrated holidays module to MarianumConnect API, replaced local Holiday model.

lib/state/app/modules/app_modules.dart

@@ -115,7 +115,7 @@ class AppModule {
       Modules.holidays: AppModule(
         Modules.holidays,
         name: 'Schulferien',
-        icon: () => Icon(Icons.flight),
+        icon: () => Icon(Icons.beach_access_outlined),
         breakerArea: BreakerArea.more,
         create: HolidaysView.new,
       ),