👔(backend) add css and code in template serializer

We want to be able to update the css and code of
a template from the update endpoint.
This commit adds the css and code fields to the
TemplateSerializer.

src/backend/core/api/serializers.py

@@ -145,9 +145,19 @@ class TemplateSerializer(BaseResourceSerializer):
 
     class Meta:
         model = models.Template
-        fields = ["id", "title", "code_editor", "accesses", "abilities"]
+        fields = ["id", "title", "code_editor", "accesses", "abilities", "css", "code"]
         read_only_fields = ["id", "accesses", "abilities"]
 
+    def to_representation(self, instance):
+        """
+        Modify the output of serialization.
+        """
+        representation = super().to_representation(instance)
+        # Remove 'css' and 'code' from the representation
+        representation.pop("css", None)
+        representation.pop("code", None)
+        return representation
+
 
 # pylint: disable=abstract-method
 class DocumentGenerationSerializer(serializers.Serializer):