invite: move invites link to the after_confirmation callback

Refactored from 6a69d958da

The `after_confirmation_path_for` isn't really made to be a callbback.
For instance, it is not executed during tests.

Moving the invitations linking to a proper documented callback allows
the linking to work in a testing environment, when invoking `user.confirm`.

config/routes.rb

@@ -70,8 +70,7 @@ Rails.application.routes.draw do
   devise_for :users, controllers: {
     sessions: 'users/sessions',
     registrations: 'users/registrations',
-    passwords: 'users/passwords',
-    confirmations: 'users/confirmations'
+    passwords: 'users/passwords'
   }
 
   devise_scope :user do