Send messages with only a CC

Summary: This keeps people in the correct To or CC field on multiplexed messages.

Test Plan:
with multiplexing on, checked that I received an email with me in the CC
field instead of the To field for a diff I'm CC'd on.

Reviewers: epriestley, jungejason, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2999