Mostly implement order refunds and cancellations

Summary:
Ref T2787. This has some rough edges but basically works.

• Users can cancel orders that are in incomplete states (or in complete states, if the application allows them to -- for example, some future application might allow cancellation of billed-but-not-shipped orders).
• Merchant controllers can partially or fully refund orders from any state after payment.

Test Plan: This is still rough around the edges, but issued Stripe and WePay refunds.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10664