Use monospaced font in Passphrase "Reveal Secret" dialog

Summary: Fixes T10812. Make it easier to disambiguate great passwords like iI|l1oO()thenumber1nospellitout.

Test Plan: {F1219074}

Reviewers: chad, yelirekim

Reviewed By: yelirekim

Maniphest Tasks: T10812

Differential Revision: https://secure.phabricator.com/D15715