don't show an opt-in button when platform isn't in signin_platforms
don't show an opt-in button when platform isn't in signin_platforms

studio666/gratipay.com,eXcomm/gratipay.com,studio666/gratipay.com,eXcomm/gratipay.com,gratipay/gratipay.com,studio666/gratipay.com,eXcomm/gratipay.com,gratipay/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,gratipay/gratipay.com,mccolgst/www.gittip.com,mccolgst/www.gittip.com,gratipay/gratipay.com,mccolgst/www.gittip.com,studio666/gratipay.com