Herald - Add "new" field to herald

Summary: ...and surface it in all adapters except commit adapters. Values are true or false. Ref T4294

Test Plan: made a herald rule to be cc'd on new tasks. was cc'd on new tasks and not cc'd on updates to existing tasks.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4294

Differential Revision: https://secure.phabricator.com/D8142