change user_name still used as login and add display_name for dashboard view instead of username on login

backend/apps/sync/serializers.py

@@ -8,7 +8,7 @@ class CitySiteSerializer(serializers.ModelSerializer):
     # Never echo the outbound city API key back to the browser by default.
     has_api_key = serializers.SerializerMethodField()
     registered_by_name = serializers.CharField(
-        source="registered_by.user_name", read_only=True, allow_null=True
+        source="registered_by.display_name", read_only=True, allow_null=True
     )
 
     class Meta: