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/controllers/forum_topics_controller.rb

@@ -24,6 +24,8 @@ class ForumTopicsController < ApplicationController
     params[:limit] ||= 40
 
     @forum_topics = ForumTopic.paginated_search(params)
+    @read_forum_topics = @forum_topics.read_by_user(CurrentUser.user)
+
     @forum_topics = @forum_topics.includes(:creator, :updater).load if request.format.html?
     @forum_topics = @forum_topics.includes(:creator, :original_post).load if request.format.atom?