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
intentional in this case. i could move this into `get_user_meta()` (and i probably will) but there're seven places in the plugin where we just want the account code. i could modify them to use `get_user_meta()` instead but that'll generate additional unnecessary function calls.