Add a very simple `bin/policy` script for CLI policy administration

Summary:
Ref T603. I want to provide at least a basic CLI tool for fixing policy problems, since there are various ways users can lock themselves out of objects right now. Although I imagine we'll solve most of them in the application eventually, having a workaround in the meantime will probably make support a lot easier.

This implements `bin/policy show <object>`, which shows an object's policy settings. In a future diff, I'll implement something like `bin/policy set --capability view --policy users <object>`, although maybe just `bin/policy unlock <object>` (which sets view and edit to "all users") would be better for now. Whichever way we go, it will be some blanket answer to people showing up in IRC having locked themselves out of objects which unblocks them while we work on preventing the issue in the first place.

Test Plan: See screenshot.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7171