Flock: added Advisory subscription support

see https://github.com/GigaOM/legacy-pro/issues/4525
actually I tried all the suggested solutions but the checked radio button still looks a bit off. Could it be because there's already [styling for `p`](https://github.com/GigaOM/go-subscriptions/blob/flock/components/sass/_sign-up.scss#L26) in this template? I tried changing the line height of the existing `p` style but that didn't quite work either.