Use special new account link

* Redirects to GCP free trial signup afterwards
* Adds GitLab referral info

app/views/projects/clusters/gcp/login.html.haml

@@ -13,7 +13,7 @@
       = link_to @authorize_url do
         = image_tag('auth_buttons/signin_with_google.png', width: '191px')
       = s_('ClusterIntegration|or create a new')
-      = link_to('Google account', 'https://accounts.google.com/signup', target: '_blank', rel: 'noopener noreferrer')
+      = link_to('Google account', 'https://accounts.google.com/SignUpWithoutGmail?service=cloudconsole&continue=https%3A%2F%2Fconsole.cloud.google.com%2Ffreetrial%3Futm_campaign%3D2018_cpanel%26utm_source%3Dgitlab%26utm_medium%3Dreferral', target: '_blank', rel: 'noopener noreferrer')
     - else
       - link = link_to(s_('ClusterIntegration|properly configured'), help_page_path("integration/google"), target: '_blank', rel: 'noopener noreferrer')
       = s_('Google authentication is not %{link_to_documentation}. Ask your GitLab administrator if you want to use this service.').html_safe % { link_to_documentation: link }