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