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.