Add mass import of commercials

Fix #1055

config/routes.rb

@@ -98,6 +98,7 @@ Osem::Application.routes.draw do
         end
         resources :event_types
         resources :difficulty_levels
+        post 'mass_upload_commercials' => 'commercials#mass_upload'
         resources :events do
           member do
             patch :toggle_attendance