FIX: Moderator actions and small actions shouldn't prevent fully merged topics from closing (#13200)

When a topic is fully merged into another topic we close it and schedule its deleting. But, because of a bug, if the merged topic contains some moderator actions or small actions it won't be merged. This change fixes this problem.

An important note: in general, we don't want to close a topic after moving posts if it still contains some regular posts or whispers. But when we are moving posts to a private message we don't want the notice about it to be publicly visible. So we use whispers with action_code == 'split_topic' instead of small_actions in such cases and we should ignore this specific kind of whispers when decide if we should close the merged topic.

app/models/post_mover.rb

@@ -66,8 +66,6 @@ class PostMover
     Guardian.new(user).ensure_can_see! topic
     @destination_topic = topic
 
-    moving_all_posts = (@original_topic.posts.pluck(:id).sort == @post_ids.sort)
-
     create_temp_table
     delete_invalid_post_timings
     move_each_post
@@ -78,7 +76,11 @@ class PostMover
     update_upload_security_status
     update_bookmarks
 
-    if moving_all_posts
+    posts_left = @original_topic.posts
+      .where("post_type = ? or (post_type = ? and action_code != 'split_topic')", Post.types[:regular], Post.types[:whisper])
+      .count
+
+    if posts_left == 1
       close_topic_and_schedule_deletion
     end