Give applications control over Phortune cart logic

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