Add more structure to Phortune product purchasing flow

Summary:
Ref T2787. When a user purchases a product in Phortune, transition the cart through a purchased state and invoke product callbacks so applications can respond to the workflow.

Also shore up some stuff like preventing negative amounts of funding.

Test Plan: Backed an initiative and saw it show up on the initiative after completing the purcahsing workflow.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10635