Implement most of the administrative UI for approval queues

Summary:
Nothing fancy here, just:

• UI to show users needing approval.
• "Approve" and "Disable" actions.
• Send "Approved" email on approve.
• "Approve" edit + log operations.
• "Wait for Approval" state for users who need approval.

There's still no natural way for users to end up not-approved -- you have to write directly to the database.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7573