emails: include logging information in email headers.

Log the following information in email headers:

* X-Danbooru-User: the user's name and ID.
* X-Danbooru-IP: the user's IP.
* X-Danbooru-Session: the users' session ID.
* X-Danbooru-URL: the page that triggered the email.
* X-Danbooru-Job-Id: the ID of the background job that sent the email.
* X-Danbooru-Enqueued-At: when the email was queued as a background job.
* X-Danbooru-Dmail: for Dmail notifications, the link to the Dmail.
* X-Request-Id: the request ID of the HTTP request that triggered the email.

Also make it so we log an event in the APM when we send an email.

app/jobs/application_job.rb

@@ -34,7 +34,7 @@ class ApplicationJob < ActiveJob::Base
       PruneRateLimitsJob, ProcessUploadJob, RegeneratePostCountsJob,
       RegeneratePostJob, RetireTagRelationshipsJob, VacuumDatabaseJob,
       DiscordNotificationJob, BigqueryExportJob, ProcessBulkUpdateRequestJob,
-      PruneJobsJob, ActionMailer::MailDeliveryJob
+      PruneJobsJob, MailDeliveryJob
     ]
   end
 end

app/jobs/mail_delivery_job.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+# A replacement for the default ActionMailer::MailDeliveryJob that inherits from ApplicationJob, so
+# it inherits the same behavior as other jobs. It also inserts the job ID into the mail headers
+# for logging purposes.
+#
+# @see https://github.com/rails/rails/blob/main/actionmailer/lib/action_mailer/mail_delivery_job.rb
+# @see https://guides.rubyonrails.org/configuring.html#config-action-mailer-delivery-job
+# @see config/application.rb (config.action_mailer.delivery_job = "MailDeliveryJob")
+class MailDeliveryJob < ApplicationJob
+  def perform(mailer, mail_method, delivery_method, args:, kwargs: nil, params: nil)
+    mailer_class = mailer.constantize.with(params.to_h)                # mailer_class = UserMailer.with(params)
+    mail = mailer_class.public_send(mail_method, *args, **kwargs.to_h) # mail = UserMailer.welcome_user(user)
+
+    mail.headers(
+      "X-Danbooru-Job-Id": job_id,
+      "X-Danbooru-Enqueued-At": enqueued_at,
+    )
+
+    mail.send(delivery_method) # mail.deliver_now
+  end
+end