FEATURE: add suggested_topics_unread_max_days_old

This new site setting determines the maximum age of unread topics in
suggested. By default if you have any unread topics older than 90 days
they will be omitted from suggested.

This change was added for 2 reasons:

1. A performance safeguard, some users tend to collect a huge amount of
read state so it becomes super expensive to find unread

2. People who collect a large amount of unread are much more interested in
recent unread topics vs ancient unread topics, this makes suggested more
relevant

Also, this is a minor speed up for tests cause 3 expensive tests became 1.

lib/topic_query.rb

@@ -222,7 +222,15 @@ class TopicQuery
         )) unless builder.full?
 
       else
-        builder.add_results(unread_results(topic: topic, per_page: builder.results_left), :high)
+
+        builder.add_results(
+          unread_results(
+            topic: topic,
+            per_page: builder.results_left,
+            max_age: SiteSetting.suggested_topics_unread_max_days_old
+          ), :high
+        )
+
         builder.add_results(new_results(topic: topic, per_page: builder.category_results_left)) unless builder.full?
       end
     end
@@ -470,12 +478,17 @@ class TopicQuery
       .order('CASE WHEN topics.user_id = tu.user_id THEN 1 ELSE 2 END')
 
     if @user
-
       # micro optimisation so we don't load up all of user stats which we do not need
       unread_at = DB.query_single(
         "select first_unread_at from user_stats where user_id = ?",
         @user.id).first
 
+      if max_age = options[:max_age]
+        max_age_date = max_age.days.ago
+        unread_at ||= max_age_date
+        unread_at = unread_at > max_age_date ? unread_at : max_age_date
+      end
+
       # perf note, in the past we tried doing this in a subquery but performance was
       # terrible, also tried with a join and it was bad
       result = result.where("topics.updated_at >= ?", unread_at)