Add very basic `bin/auth` tool

Summary: Ref T1536. This script basically exists to restore access if/when users shoot themselves in the foot by disabling all auth providers and can no longer log in.

Test Plan: {F46411}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6205