Herald - add support for "content source" conditions

Summary: ...and deploy on Maniphest. Ref T1638.

Test Plan: created a herald rule to be cc'd for tasks created via web. made a task via web and another via email and was cc'd appropriately. edited the herald to be cc'd for tasks created via not web. made 2 tasks again and got cc'd appropriately

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T1638

Differential Revision: https://secure.phabricator.com/D7145