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