Add `bin/policy unlock`

Summary: Ref T603. We might need a fine-grained CLI tool later on, but here's a bat we can bludgeon things with.

Test Plan:
  - Ran `bin/policy unlock D12` (adjusted policies).
  - Ran `bin/policy unlock rPca85c457ebcb` (got "not mutable" stuff).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7189