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
@zbtirrell i'm thinking this should be in `go-subscriptions` as well. i moved these js here initially because they were named go-recurly-* but looking at them now i think they belong in `go-subscriptions`.