fixes #2191

app/mailers/user_mailer.rb

@@ -1,4 +1,5 @@
 class UserMailer < ActionMailer::Base
+  add_template_helper ApplicationHelper
   default :from => Danbooru.config.contact_email, :content_type => "text/html"
 
   def dmail_notice(dmail)
@@ -13,4 +14,10 @@ class UserMailer < ActionMailer::Base
   def upgrade_fail(email)
     mail(:to => "#{user.name} <#{email}>", :subject => "#{Danbooru.config.app_name} account upgrade")
   end
+
+  def forum_notice(user, forum_topic, forum_posts)
+    @forum_topic = forum_topic
+    @forum_posts = forum_posts
+    mail(:to => "#{user.name} <#{user.email}>", :subject => "#{Danbooru.config.app_name} forum topic #{forum_topic.title} updated")
+  end
 end