Fixes T13265. See that task for discussion. Briefly:
- For mailers that use other mailers (SMTP, Sendmail), optionally let administrators set "message-id": false to improve threading behavior if their local Postfix is ultimately sending through SES or some other mailer which will replace the "Message-ID" header.
- Postmark is currently marked as supporting "Message-ID", but it does not actually support "Message-ID" on secure.phabricator.com (mail arrives with a non-Phabricator message ID). I suspect this was just an oversight in building or refactoring the adapter; correct it.
- Remove the "encoding" parameter from "sendmail". It think this was just missed in the cleanup a couple months ago; it is no longer used or documented.