Freebird: the new go-recurly plugin

mostly pulled out of `go-subscriptions` and `go-subscriptions-freebies`.

see https://github.com/GigaOM/legacy-pro/issues/4125
Can you add a `@TODO` to handle this in JS instead of PHP?

Reason: we cache our content and suppress cookie setting so heavily that doing it in PHP may not be working as expected, everywhere it's expected.