Add "High Security" mode to support multi-factor auth

Summary:
Ref T4398. This is roughly a "sudo" mode, like GitHub has for accessing SSH keys, or Facebook has for managing credit cards. GitHub actually calls theirs "sudo" mode, but I think that's too technical for big parts of our audience. I've gone with "high security mode".

This doesn't actually get exposed in the UI yet (and we don't have any meaningful auth factors to prompt the user for) but the workflow works overall. I'll go through it in a comment, since I need to arrange some screenshots.

Test Plan: See guided walkthrough.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4398

Differential Revision: https://secure.phabricator.com/D8851