Payment Gateway API for Leaky Paywall

1. Create subclass
– set up test mode and supports
– load any dependencies
– process signup
– process webhooks
– add fields to sign up form
– validate fields
– load any checkout scripts

2. Register the gateway (adds to the list of possible gateways in Leaky Paywall settings)

3. Add sign up link

4. Add gateway settings to Leaky Paywall page

5. Add slug filter so that it shows up in the subscriber table