Move Phortune product logic into applications

Summary: Ref T2787. Product is currently a fairly heavy object, but as Phortune develops it makes a lot of sense to make it a lighter object and put more product logic in applications. Convert it into a fairly lightweight reference to applications. The idea is that Phortune is mostly providing a cart flow, and applications manage the details of products.

Test Plan: Funded an initiative for $1.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10634