Write features for gocardless webhooks; paid, failed, cancelled, and expired.
Add gocardless webhook

And test steps
Handle subscription cancellation and expiry

Resolves #4
Add payment model
Test pod visibility based on payments separately
Add payment webhook handling
Handle chargebacks
Refactor payment fields

To be able to more accurately track state
Add payment create and retry support

So we can easily see when things are taking place. This lets us, for instance, grant access to content earlier.
Refactor some webhook logic into subscription
Refactor features into separate files
Merge pull request #19 from seedpod/webhooks

Webhooks