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
If we wanted to use `wp_kses`, it throws a notice if the second parameter isn't passed.  `wp_kses( $this->recurly_error->getMessage(), array() )`.

That being said, I agree that `wp_kses` might be overkill.  Is the error message just text? If so, `esc_html` might be the better option.