add default entries creation on post-migrate signal and refactor application model logic

app_gateway/apps.py

@@ -4,3 +4,8 @@ from django.apps import AppConfig
 class AppGatewayConfig(AppConfig):
     default_auto_field = 'django.db.models.BigAutoField'
     name = 'app_gateway'
+
+    def ready(self):
+        from django.db.models.signals import post_migrate
+        from app_gateway.setup_defaults import create_default_entries
+        post_migrate.connect(create_default_entries, sender=self)