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
89c66cbbd4