Disable Devise sign ups if Guisso is enabled

If we offload authorisations to Guisso, then avoid local users sign ups.
Guisso is CAPTCHA-protected, so we'd rather use it for production than
add CATPCHA protections to each InSTEDD app.

config/routes.rb

@@ -30,7 +30,7 @@
 
   match '/' => 'home#index',  :as => 'home'
 
-  devise_for :accounts, controllers: {omniauth_callbacks: "omniauth_callbacks" , sessions: "sessions"}
+  devise_for :accounts, :skip => [ ( :registrations if Guisso.enabled? ) ], controllers: {omniauth_callbacks: "omniauth_callbacks" , sessions: "sessions"}
   guisso_for :account
 
   resources :feeds, controller: :feed_server do