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