Clean up some Passphrase transaction bugs

Summary: Ref T12685. Makes the description field full remarkup and fixes setting a credential secret after destruction.

Test Plan: Change description a lot, set and destroy credentials.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12685

Differential Revision: https://secure.phabricator.com/D17851