Proposal for widening use case to GoCardless (and others)

After a lot of research and discussion with @mattwire, I think this is the way to go.

1. changes schema (upgrader included): adding indexes, fields, modifying existing fields to be more efficent, using 'new' not NULL for status.
2. a PaymentprocessorWebhook record describes and event, not the contents of a webhook. These are one and the same in systems where one event = one webhook, but for others (e.g. GoCardless) this is an important distiction.
3. To implement this, Payment processor extensions only need to tweak their handlePaymentNotification method, and add a new processWebhookEvent() method.
4. Existing Stripe support (legacy code) has been kept for backwards compat.
5. Docs updated a lot, with sample/pseudo code.