Stripe Custom accounts

[Changaco]

This issue is like #2238 but for Custom rather than Express accounts. The main difference between Express and Custom is that with the latter the user wanting to collect donations doesn't go through Stripe's website at all. Adding support for Custom accounts requires even more work than for Express accounts.

[ItsJustTee]

Hey Changaco, just a fair warning before proceeding with Stripe Custom integration. South African banks are starting to not accept International Transfers anymore, thus you would need to enable Electronic Funds Transfers for Payouts on Stripe to successfully completed. I was given the same warning by my Bank.

[Changaco]

Platforms can now white label payment workflows in record time

The new components provided by Stripe should enable implementing support for Custom accounts relatively quickly.