Improve the usability of Phortune

Summary:
Ref T6881.

• Fix dead links.
• Let implementations provide more information.
• Provide more information to implementations.

Test Plan: Links work, invoices show billing periods, fewer "Subscription 6" crumbs, all is well in the world.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11601