Phortune Carts and Purchases

Summary:
Ref T2787. Make carts and purchases real objects, with storage, that kind-of work.

Roughly, the idea here is that applications create "purchases" (like "1 large t-shirt") and add them to "carts" (a user can have a lot of different carts at the same time), then hand things off to Phortune to deal with actualy charging a card. Roughly this works like Paypal or other similar systems do, except Phortune is the thing the user gets handed off to.

This doesn't do anything interesting/useful yet.

Also fix some bugs and update some UI.

Test Plan: Added a product to a cart, saw it in cart screen.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10001