R1:49ef13e876fd

Add WePay as a one-time payment provider

Summary:
Ref T2787.

I //think// we could also use WePay as a recurring payment provider, but this is somewhat messy (OAuth + requires account) -- basically it's "add a WePay account" instead of "add a credit card".

The WePay checkout workflow is a bit upsell-y but basically reasonable.

I like that their API just has a `request($method, $params)` method instead of 30,000 lines of methods for each request type. I did hit one bug; I'll send a pull for that.

Test…