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.
  - Hit all failure states where we just dead-end the cart. Not ideal, but (seemingly) complete/safe/correct.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10639