implemented the is_signup() function

in support of `go-local-subscriptions`'s ribbon widget

should be merged at the same time as:
* https://github.com/GigaOM/gigaom/pull/5354
* https://github.com/GigaOM/gigaom-plugins/pull/2684
* https://github.com/GigaOM/go-local-subscriptions/pull/1
* https://github.com/GigaOM/go-recurly/pull/21

as part of https://github.com/GigaOM/legacy-pro/issues/4055