Add admin-only inventory views to facilitate drilling down

[mathuin]

Right now there's a little bit of this in the admin site, but there's a lot of potential for a proper inventory view which can show every jar in every crate in every bin on every shelf in every row in every warehouse, all collapsed via CSS or something. This would make inventory auditing much, much easier, as well as possibly provide a visual reference for empty or near-empty crates.