Update the Phortune cart/invoice workflow for policy changes

Summary:
Depends on D20734. Ref T13366. This makes the cart/order flow work under the new policy scheme with no "grantAuthority()" calls.

It prepares for a "Void Invoice" action, although the action doesn't actually do anything yet.

Test Plan: With and without merchant authority, viewed and paid invoices and went through the other invoice interaction workflows.

Maniphest Tasks: T13366

Differential Revision: https://secure.phabricator.com/D20735