[#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
- Order by amount to ensure smallest variants are re-used
- Allow setting memo/description field
- Enforce no <= $0.00 payments via form, not PGProvider

Story: https://www.pivotaltracker.com/story/show/82259176