R1:7720b799e45c

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

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