Passphrase v0

Summary:
Ref T4122. Implements a credential management application for the uses described in T4122.

@chad, this needs an icon, HA HA HAHA HA BWW HA HA HA

bwahaha

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T4122

Differential Revision: https://secure.phabricator.com/D7608