[Fix #2068] Use mail templates from application rather than devise

`default template_path:` was the documented way, but wasn’t working
(and there was no hint in the source code of Devise that it could
work)

Therefore, let’s override `template_paths` instead and stop worrying

app/mailers/devise_user_mailer.rb

@@ -1,6 +1,9 @@
 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'
+
+  def template_paths
+    ['devise_mailer']
+  end
 end