Gateway transcript scrubbing

This is a first pass at adding transcript scrubbing to ActiveMerchant gateways - here's what I've done:

1) Implemented scrubbing for Stripe
2) Implemented scrubbing for PayPal
3) Added a stub `Gateway#scrub` method to the Gateway template, as well as a remote test that will automatically write out the transcript for a purchase call and then fail (so that you can fill in scrubbing for the gateway you're implementing).

This PR doesn't change access to the transcripts, because ActiveMerchant isn't actually capturing them (they are only captured in the one remote test, in order to be used to write your scrubbing code) - if you want to capture and then scrub them, you have to do that part yourself (but you can tell if a Gateway *can* scrub something by checking if it `respond_to?(:scrub)`). If someone is writing a new integration and doesn't feel like writing the scrubbing logic they can just remove the stubs.

@j-mutter @bizla @Shopify/active-merchant