Fix an issue with To/CC placeholders

Summary: Currently, if no placeholder is configured we always move "Cc" up to "To", even if we have a valid "To". Instead, move "Cc" to "To" only if there's no "To" and no placeholder.

Test Plan: Sent email with "to" and "cc", email with "cc" only with a placeholder, and email with "cc" only without a placeholder. Verified recipients ended up in the right location in all cases.

Reviewers: nh, btrahan

Reviewed By: btrahan

CC: klimek, aran

Differential Revision: https://secure.phabricator.com/D3342