FEATURE: Automatically timed delete stub topics after entire topic is merged into another topic (#13187)

When a topic is fully merged into another topic we close it. Now we want also to set a timer for deleting this topic. By default, stub topics will be deleted in 7 days. Users can change this period or disable auto-deleting by setting the period to 0.

app/models/post_mover.rb

@@ -79,7 +79,7 @@ class PostMover
     update_bookmarks
 
     if moving_all_posts
-      @original_topic.update_status('closed', true, @user)
+      close_topic_and_schedule_deletion
     end
 
     destination_topic.reload
@@ -546,4 +546,17 @@ class PostMover
       topic_id: topic.id
     )
   end
+
+  def close_topic_and_schedule_deletion
+    @original_topic.update_status('closed', true, @user)
+
+    days_to_deleting = SiteSetting.delete_merged_stub_topics_after_days
+    if days_to_deleting > 0
+      @original_topic.set_or_create_timer(
+        TopicTimer.types[:delete],
+        days_to_deleting * 24,
+        by_user: @user
+      )
+    end
+  end
 end