Allow linking to passphrase credential via remarkup

Summary: Fixes T5421, add linking to passphrase credentials

Test Plan: Open task, add a comment of '{K1}' where K1 is a passphrase credential. Preview and actual comment should be link to credential with the content of credential.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5421

Differential Revision: https://secure.phabricator.com/D9725