FIX: Dismiss new with better migration (#12062)

Original PR was reverted because of broken migration https://github.com/discourse/discourse/pull/12058

I fixed it by adding this line
```
          AND topics.id IN(SELECT id FROM topics ORDER BY created_at DESC LIMIT :max_new_topics)
```

This time it is left joining a limited amount of topics. I tested it on few databases and it worked quite smooth

app/services/dismiss_topics.rb

@@ -8,6 +8,7 @@ class DismissTopics
 
   def perform!
     DismissedTopicUser.insert_all(rows) if rows.present?
+    @rows.map { |row| row[:topic_id] }
   end
 
   private
@@ -17,6 +18,7 @@ class DismissTopics
       .joins("LEFT JOIN topic_users ON topic_users.topic_id = topics.id AND topic_users.user_id = #{@user.id}")
       .where("topics.created_at >= ?", since_date)
       .where("topic_users.id IS NULL")
+      .where("topics.archetype <> ?", Archetype.private_message)
       .order("topics.created_at DESC")
       .limit(SiteSetting.max_new_topics).map do |topic|
       {
@@ -38,6 +40,6 @@ class DismissTopics
       else
         new_topic_duration_minutes.minutes.ago
       end
-    [setting_date, @user.user_stat.new_since, Time.at(SiteSetting.min_new_topics_time).to_datetime].max
+    [setting_date, @user.created_at, Time.at(SiteSetting.min_new_topics_time).to_datetime].max
   end
 end

app/services/user_merger.rb

@@ -266,6 +266,8 @@ class UserMerger
     update_user_id(:draft_sequences, conditions: "x.draft_key = y.draft_key")
     update_user_id(:drafts, conditions: "x.draft_key = y.draft_key")
 
+    update_user_id(:dismissed_topic_users, conditions: "x.topic_id = y.topic_id")
+
     EmailLog.where(user_id: @source_user.id).update_all(user_id: @target_user.id)
 
     GroupHistory.where(acting_user_id: @source_user.id).update_all(acting_user_id: @target_user.id)