Summary: Ref T2787. Similar to D10634, give applications more control over the cart workflow. For now this just means they get to pick exit URIs, but in the future they can manage more details of cart behavior.
Test Plan: Funded an initiative and got returned to the initiative instead of dead-ending in Phortune.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T2787
Differential Revision: https://secure.phabricator.com/D10638
