Update Phortune subscriptions for modern infrastructure

Summary:
Depends on D20720. Ref T13366.

• Use modern policies and policy interfaces.
• Use new merchant authority cache.
• Add (some) transactions.
• Move MFA from pre-upgrade-gate to post-one-shot-check.
• Simplify the autopay workflow.
• Use the "reloading arrows" icon for subscriptions more consistently.

Test Plan: As a merchant-authority and account-authority, viewed, edited, and changed autopay for subscriptions.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13366

Differential Revision: https://secure.phabricator.com/D20721