Allow "SMTP" and "Sendmail" mailers to have "Message-ID" behavior configured in "cluster.mailers"
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.
Test Plan: Added and ran unit tests. (These feel like overkill, but this is super hard to test on real code.) See T13265 for evidence that this overall approach improves behavior.
Reviewed By: amckinley
Maniphest Tasks: T13265
Differential Revision: https://secure.phabricator.com/D20285