Make Herald rules subscribable

Summary: Fixes T9757.

Test Plan: Created a Herald rule and then subscribed to it with a different account.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T9757

Differential Revision: https://secure.phabricator.com/D14468