Add topic.posts_count to web_hook_posts_serializer

app/serializers/web_hook_post_serializer.rb

@@ -1,4 +1,7 @@
 class WebHookPostSerializer < PostSerializer
+
+  attributes :topic_posts_count
+
   def include_topic_title?
     true
   end
@@ -14,4 +17,9 @@ class WebHookPostSerializer < PostSerializer
       false
     end
   end
+
+  def topic_posts_count
+    object.topic && object.topic.posts_count
+  end
+
 end