WIP redirect favicon.ico requests to www.gov.uk/favicon.ico

- now that static isn't serving favicon.ico, we need to serve it
  on assets.publishing.service.gov.uk somehow, this seems
  the simplest solution.
- TODO: tests, redirect to appropriate environment?

config/routes.rb

@@ -8,6 +8,8 @@
     Healthcheck::CloudStorage,
   )
 
+  get "/favicon.ico" => redirect("https://www.gov.uk/favicon.ico")
+
   resources :assets, only: %i[show create update destroy] do
     member do
       post :restore