Enable ActionMailer previews in staging

staging.rb

Rails::Application.configure do
  #########################
  # Other config settings #
  #########################
  
  # Enable ActionMailer previews in staging
  config.action_mailer.preview_path ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil
  config.autoload_paths += [config.action_mailer.preview_path]
  routes.append do
    get '/rails/mailers'         => "rails/mailers#index"
    get '/rails/mailers/*path'   => "rails/mailers#preview"
  end
end

class ::Rails::MailersController
  include Rails.application.routes.url_helpers
  def local_request?
    true
  end
end