FEATURE: Add topic and category context to post webhook payload (#8110)

Adds additional fields topic_archetype and category_slug to the post
webhook so that handlers have some context about the post event without
having to call back to the API.

Discussed [here](https://meta.discourse.org/t/webhooks-how-best-to-differentiate-a-pm-from-a-public-post/76363/13).

app/serializers/web_hook_post_serializer.rb

@@ -2,7 +2,9 @@
 
 class WebHookPostSerializer < PostSerializer
 
-  attributes :topic_posts_count
+  attributes :topic_posts_count,
+             :topic_archetype,
+             :category_slug
 
   def include_topic_title?
     true
@@ -32,6 +34,18 @@ class WebHookPostSerializer < PostSerializer
     object.topic ? object.topic.posts_count : 0
   end
 
+  def topic_archetype
+    object.topic ? object.topic.archetype : ''
+  end
+
+  def include_category_slug?
+    object.topic && object.topic.category
+  end
+
+  def category_slug
+    object.topic && object.topic.category ? object.topic.category.slug_for_url : ''
+  end
+
   def include_readers_count?
     false
   end