Include the Namespace Id in purchase link for GL.com plans

This will automatically select the respective User or Group in the
purchase form used in the subscription portal.

ee/app/views/shared/billings/_billing_plan.html.haml

@@ -30,7 +30,7 @@
 
     - if purchase_link
       .plan-action
-        - href = purchase_link.href
+        - href = purchase_link.href&.concat("&gl_namespace_id=#{namespace.id}")
 
         - case purchase_link.action
         - when 'downgrade'