Make Passphrase Credentials Flaggable

Summary: Ref T8888, Makes Passphrase credentials flaggable.

Test Plan: Flag a credential

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8888

Differential Revision: https://secure.phabricator.com/D13655