Add boilerplate scaffolding for Phortune subscriptions

Summary:
Ref T6881. This roughs in the major objects, support classes, and controllers.

• Show subscriptions on account detail.
• Browse all account subscriptions.
• Link to active subsciptions from merchant detail.

Test Plan: Clicked around in the UI. There's no way to create subscriptions yet, so I basically just kicked the tires on this. I probably missed a few things that I'll clean up in followups.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11482