demarches-normaliennes/app/mailers/devise_user_mailer.rb

class DeviseUserMailer < Devise::Mailer
  helper :application # gives access to all helpers defined within `application_helper`.
  include Devise::Controllers::UrlHelpers # Optional. eg. `confirmation_url`
  default template_path: 'devise_mailer' # to make sure that your mailer uses the devise views
  layout 'mailers/layout'
end