Replace "Show Secret" in Passphrase with one-shot MFA

Summary: Depends on D20036. Ref T13222. Now that we support one-shot MFA, swap this from session MFA to one-shot MFA.

Test Plan: Revealed a credential, was no longer left in high-security mode.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D20037