R1:c694bd651b49

Make base64 the default SMTP encoding

Summary: Ref T11120. If this works, I'll just remove this option completely.

Test Plan: ¯\_(ツ)_/¯

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11120

Differential Revision: https://secure.phabricator.com/D16095