add link to adherer form to bypass google bug

sometimes, you have a google account cookie but google wants you to sign in again to verify your password, for example if you don't have used this cookie in a while.
This problem impact us because google ask this on every google property, even if you're consulting a document which doesn't require signing in, like our sign-up form (using google forms)
Furthermore, google doesn't load the sign-in form in iframe, to prevent spoofing/phishing, so the adherer page appear blank