R1:c0e1a63a63ae

Implement an approval queue

Summary:
- Add an option for the queue.
- By default, enable it.
- Dump new users into the queue.
- Send admins an email to approve them.

Test Plan:
- Registered new accounts with queue on and off.
- As an admin, approved accounts and disabled the queue from email.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7576