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
this is a really good point. the `images/...` files are from recurly-js's theme directory, but the `img/...` files are from our components directory. this made me realize that i didn't move at least one of the images in `img/` from `go-subscriptions` to `go-recurly`.