Add an after-purchase hook to subscriptions in Phortune

Summary:
Ref T12681. We need this to update the "paid until" window on support pacts.

(Instance billing doesn't use this because everything just checks if you have unpaid invoices, nothing actually happens when you pay them.)

Test Plan: See D18187.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12681

Differential Revision: https://secure.phabricator.com/D18188