[#82259176] Past variant lookup improvements, capture_via_cc with amount

- Allow past variant lookup by pg_authorization_code
- `capture_via_cc` now requires an `amount` arg in case amount changes between auth/capture

Story: https://www.pivotaltracker.com/story/show/82259176

Assigned @dlinder 
Seems your life would be easier here, nesting-wise, if you hoisted the PaymentFailure up here and reversed the condition.