forum: optimize unread forum topics on forum index.

Avoid doing one SQL query per topic when checking for new topics on the
forum index.

This also changes it so that forum topics aren't always marked as new
for anonymous users.

app/helpers/forum_topics_helper.rb

@@ -6,4 +6,8 @@ module ForumTopicsHelper
   def available_min_user_levels
     ForumTopic::MIN_LEVELS.select { |name, level| level <= CurrentUser.level }.to_a
   end
+
+  def new_forum_topic?(topic, read_forum_topics)
+    !read_forum_topics.map(&:id).include?(topic.id)
+  end
 end