Don't send email to bots subscribed to diffs

Summary:
Occasionally a bot will get subscribed to a differential revision; when
this happens we shouldn't send them an email, because otherwise the
author of the diff will get a bounce email if the bot has an invalid
emal address.

Test Plan:
re-sent a message that had a bot on the cc list, saw that it was no longer
included when it got sent.

Reviewers: epriestley, jungejason, vrana

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2479