Clean up numerous rough edges in Mail configuration
Summary:
- Support file attachments in Mailgun, after D8831.
- Fix bin/mail send-test --attach ... flag.
- Make bin/mail send-test route mail through the daemons.
- Remove the workerTaskID on MetaMTAMail, which is only used (needlessly) by bin/mail resend and creates a huge mess elsewhere.
- Currently, when mail fails, the daemon exits with a very generic and useless message. Instead, make sendNow() throw when it fails, so the real reason is surfaced. This is OK now because mail is always sent via the daemons.
- Now that Mailgun supports attachments, document it.
- Update a bunch of mail docs.
Test Plan:
- Sent mail.
- Sent mail with attachments.
- Read documentation.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Differential Revision: https://secure.phabricator.com/D8832