emails: fix one-click unsubscription.

Fix it so that emails are (hopefully) able to show the one-click
unsubscribe button in Gmail and other mail providers that support the
List-Unsubscribe header. This way users can unsubscribe instead of
marking emails as spam.

* Add the List-Unsubscribe-Post header.
* Fix the disable email notifications endpoint to support POST as well as DELETE requests.
* Fix the disable email notifications endpoint to disable XSRF protection (we don't need
  users to be logged in because we use a signed URL instead).

https://www.rfc-editor.org/rfc/rfc8058#section-3.1
https://www.rfc-editor.org/rfc/rfc8058#section-8.1

config/routes.rb

@@ -49,7 +49,7 @@ Rails.application.routes.draw do
   namespace :maintenance do
     namespace :user do
       resource :count_fixes, only: [:new, :create]
-      resource :email_notification, :only => [:show, :destroy]
+      resource :email_notification, only: [:show, :create, :destroy]
       resource :deletion, :only => [:show, :destroy]
     end
   end