Fix AccessDenied error handler

I don't think people see this very often visually, but it's showing up
in our error logging! The Rails API changed here long ago and we didn't
notice: to render public files, we should use the `file` argument
instead of `template`.

app/controllers/application_controller.rb

@@ -57,7 +57,7 @@ class ApplicationController < ActionController::Base
   rescue_from AccessDenied, :with => :on_access_denied
 
   def on_access_denied
-    render template: 'public/403.html', :layout => false, :status => :forbidden
+    render file: 'public/403.html', layout: false, status: :forbidden
   end
 
   def redirect_back!(default=:back)