Retry mailer jobs on common networking and SMTP errors

This concern is lifted nearly verbatim from Basecamp.

ref: https://app.fizzy.do/5986089/cards/3300

lib/rails_ext/action_mailer_mail_delivery_job.rb

@@ -0,0 +1,3 @@
+Rails.application.config.to_prepare do
+  ActionMailer::MailDeliveryJob.include SmtpDeliveryErrorHandling
+end