R1:fcf5149b3648

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…