FEATURE: new setting to create a linked topic on autoclosing mega topics (#11001)

This commit adds a site setting `auto_close_topics_create_linked_topic`
which when enabled works in conjunction with `auto_close_topics_post_count`
setting and creates a new linked topic for the topic just closed.

The auto-created new topic contains a link for all the previous topics
and the topic titles are appended with `(Part {n})`.

The setting is enabled by default.

lib/post_creator.rb

@@ -381,6 +381,11 @@ class PostCreator
           locale: SiteSetting.default_locale
         )
       )
+
+      if SiteSetting.auto_close_topics_create_linked_topic?
+        # enqueue a job to create a linked topic
+        Jobs.enqueue_in(5.seconds, :create_linked_topic, post_id: @post.id)
+      end
     end
   end