PERF: Keep track of when a users first unread is

This optimisation avoids large scans joining the topics table with the
topic_users table.

Previously when a user carried a lot of read state we would have to join
the entire read state with the topics table. This operation would slow down
home page and every topic page. The more read state you accumulated the
larger the impact.

The optimisation helps people who clean up unread, however if you carry
unread from years ago it will only have minimal impact.

lib/topic_query.rb

@@ -469,6 +469,10 @@ class TopicQuery
         staff: @user&.staff?)
       .order('CASE WHEN topics.user_id = tu.user_id THEN 1 ELSE 2 END')
 
+    if @user
+      result = result.where("topics.updated_at >= (SELECT first_unread_at FROM user_stats WHERE user_id = ?)", @user.id)
+    end
+
     self.class.results_filter_callbacks.each do |filter_callback|
       result = filter_callback.call(:unread, result, @user, options)
     end