Fix #4515: Set List-Unsubscribe header on notification emails.

app/mailers/application_mailer.rb

@@ -4,9 +4,14 @@
 #
 # @see https://guides.rubyonrails.org/action_mailer_basics.html
 class ApplicationMailer < ActionMailer::Base
+  helper :application
+  helper :users
+  include UsersHelper
+
   default from: "#{Danbooru.config.canonical_app_name} <#{Danbooru.config.contact_email}>", content_type: "text/html"
 
   def mail(user, require_verified_email:, **options)
+    headers["List-Unsubscribe"] = disable_email_notifications_url(user)
     message = super(to: user.email_address&.address, **options)
     message.perform_deliveries = user.can_receive_email?(require_verified_email: require_verified_email)
     message

app/mailers/user_mailer.rb

@@ -1,9 +1,6 @@
 # frozen_string_literal: true
 
 class UserMailer < ApplicationMailer
-  helper :application
-  helper :users
-
   # The email sent when a user receives a DMail.
   def dmail_notice(dmail)
     @dmail = dmail