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
i think i know what you mean. the signup action callback in go-recurly will lazily instantiate go-recurly-freebies only when the invite link is clicked. the bulk of the signup code is still here at the same place.