FIX: Don't update posts_read_count when the post is from a PM. (#12131)

We don't want TL0 users doing the discobot tutorial to increase their read count.

app/models/post_timing.rb

@@ -40,6 +40,8 @@ class PostTiming < ActiveRecord::Base
     # still happen, if it happens we just don't care, its an invalid record anyway
     return if row_count == 0
     Post.where(['topic_id = :topic_id and post_number = :post_number', args]).update_all 'reads = reads + 1'
+
+    return if Topic.exists?(id: args[:topic_id], archetype: Archetype.private_message)
     UserStat.where(user_id: args[:user_id]).update_all 'posts_read_count = posts_read_count + 1'
   end