R1:fcd2025a85ba

Add Merchants to Phortune

Summary:
Ref T2787. Currently, you add payment providers (Stripe, Paypal, etc) in global configuration.

Generally, this approach is cumbersome, limiting, and often hard for users to figure out. It also doesn't provide a natural way to segment payment receivers or provide web access to administrative payment functions like issuing refunds, canceling orders, etc. I think that stuff definitely needs to be in the web UI, and the rule for access to it can't reasonably just be "all…