Print robots tag for registration layout

This is copied from the darkswarm layout.

app/views/layouts/registration.html.haml

@@ -2,6 +2,8 @@
   %head
     %meta{charset: 'utf-8'}/
     %meta{name: 'viewport', content: "width=device-width,initial-scale=1.0"}/
+    - if !Rails.env.production? || @noindex_meta_tag
+      %meta{name: "robots", content: "noindex"}
 
     %title= content_for?(:title) ? "#{yield(:title)} - #{Spree::Config[:site_name]}".html_safe : "#{t(:welcome_to)} #{Spree::Config[:site_name]}"
     - if Rails.env.production?