Adjust payment workflows to deal with merchants and configurable providers in Phortune

Summary:
Ref T2787. Builds on D10649 by rebining existing objects (carts, charges, etc) to merchantPHIDs and providerPHIDs instead of an implicit global merchant and weird global artifacts (providerType / providerKey).

Basically:

  - When you create something that users can pay for, you specify a merchant to control where the payment goes.
  - Accounts are install-wide, but payment methods are bound to merchants. This seems to do a reasonable job of balancing usability and technical concerns.
  - Replace a bunch of weird links between objects with standard PHIDs.
  - Improve "add payment method" flow.

Test Plan: Went through the Fund flow with Stripe and WePay, funding an initiative.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10652