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