link to submission preview page in completed emails

config/routes.rb

@@ -80,6 +80,8 @@ Rails.application.routes.draw do
     get :completed
   end
 
+  resources :submissions_preview, only: %i[show], path: 'e', param: 'slug'
+
   resources :send_submission_email, only: %i[create] do
     get :success, on: :collection
   end