Fix route and error on trial_registrations/new endpoint

Original route was using an extra parameter (/-/...). Also the endpoint
was expecting the presence of the `user` param.

config/routes.rb

@@ -193,6 +193,7 @@ Rails.application.routes.draw do
   draw :project
 
   Gitlab.ee do
+    draw :trial_registration
     draw :trial
   end