Add an "Explicit-CCs" header to Differential

Summary: This header allows recipients to distinguish between CCs generated by Herald and CCs generated by humans.

Test Plan: Created a Herald rule to add a bunch of CC's to every revision. Created a revision. Added some CCs manually. Verified that only manual CCs appeared in the "Explicit" header.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T808

Differential Revision: https://secure.phabricator.com/D2018