R1:4ef547f8d6db

Give WePay complete payment logic in Phortune

Summary:
Ref T2787. This doesn't get all the edge cases quite correct, but is generally a safe, complete payment workflow:

- Shares the actual charging state logic.
- Makes it appropriately stateful with locking and transactions.
- Gets the main flow correct.
- Detects failure cases, just tends to blow up rather than help the user resolve them.

Test Plan:
- Charged with WePay.
- Charged with Infinite Free Money.
- Resumed an abandoned cart.