PERF: Exclude `image_url` and `thumbnails` from `SearchTopicListItemSerializer`.

The attributes are not used and was resulting in N+1 queries.

app/serializers/search_topic_list_item_serializer.rb

@@ -4,4 +4,12 @@ class SearchTopicListItemSerializer < ListableTopicSerializer
   include TopicTagsMixin
 
   attributes :category_id
+
+  def include_image_url?
+    false
+  end
+
+  def include_thumbnails?
+    false
+  end
 end