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