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 started looking into this but decided to put it off. it's not a simple matter of upgrading Recurly's js library -- the v3 API is not compatible with v2 so we'll need to rewrite much of our code to work with v3. I think we should put this off as a separate task from Freebird. (They still support v2 -- it just won't have some of the new features in v3.)