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.

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