Give "MetaMTAMail" a "message type" and support SMS

Summary:
Depends on D20011. Ref T920. This change lets a "MetaMTAMail" storage object represent various different types of messages, and makes "all" the bin/mail stuff "totally work" with messages of non-email types.

In practice, a lot of the related tooling needs some polish/refinement, but the basics work.

Test Plan: Used echo beep boop | bin/mail send-test --to epriestley --type sms to send myself SMS.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D20012