don't add email addresses to CC if they are already in TO

Summary:
Some mailers remove the duplicate entries themselves, but some (Mailgun) don't.
This affects installations with metamta.one-mail-per-recipient set to false, and will cause

• ugly looking "to" entries. Gmail, for example, collapses to+cc entries to one list, so you get something that looks like "to: me me john"
• It sometimes causes duplicate delivery of the same message when used in conjuction with Google Groups. I suspect that their message de-dup mechanism is confused by it (I fuzzed it directly with Mailgun, and saw the same message delivered twice - once directly through mailgun, and bounced again through Google Groups). This doesn't happen when the entries are not duplicated.

Test Plan: Created some tasks. Added subscribers. Things seem to work reasonably well.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9978