[herald] Add support for Arcanist Project as a field for Differential revisions

Summary: Useful in cases where there is an Arcanist Project but not a repository tracked by Phabricator for a particular revision.

Test Plan: Created a new rule to flag Differential revisions with a particular Arcanist project, verified that it applied as expected via the test console to revisions with the project specified and with a different project specified.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8463