Add Subscribers to Passphrase

Summary: Fixes T9078, Adds SubscribableInterface to Passphrase.

Test Plan: Create a new passphrase, see myself subscribed. Subscribe to other Passphrases.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9078

Differential Revision: https://secure.phabricator.com/D13799