Investigate to merge Charge.payment_successful on checkout request in RazorPay setup

[smirolo]

The flow to subscribe and pay for a plan is as follow:

1. POST to /api/cart/
2. GET /api/billing/:organization/checkout/
3. compute the total amount to charge from the list of invoicables.
4. POST to RazorPay
5. POST /api/billing/:organization/checkout/ with {“processor_token”: token-returned-by-razorpay}
   This will call RazorpayBackend.create_charge_on_card to capture the total amount of invoicables.
6. GET /api/billing/charges/:charge (:charge being return by the previous call as "processor_key")
   This will call Charge.payment_successful and create the Transaction in the database as well as switch Charge.state from "created" to “done".

RazorPay works somewhat differently from Stripe. The processor_token passed back by RazorPay is an actual charge id. The processor_token passed back by Stripe is a card id. It is quite possible that payment_successful could be called after razorpay.capture and make step #6 optional in RazorPay setup.