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 actually can't find any references to "research-generic-button", nor "research-generic" nor "generic-button" in themes, mu-plugins or plugins. these might not be used any more. i guess i can remove and see if that breaks anything.