PERF: topic_view participant post count: don't send back ID list (#10210)

On large topics, the cost of sending the entire post ID list back over to the database is signficant. Just have the DB recalculate the list of visible posts instead.

lib/guardian/topic_guardian.rb

@@ -31,6 +31,10 @@ module TopicGuardian
     is_staff? && SiteSetting.enable_whispers?
   end
 
+  def can_see_whispers?(_topic)
+    is_staff?
+  end
+
   def can_publish_topic?(topic, category)
     is_staff? && can_see?(topic) && can_create_topic?(category)
   end