You can add the code (without the <script> tags) to your theme's 'custom.js' JavaScript file and it will be included (and run) in all your Showkase pages.
For example, if you use the Boma theme, then add the following code (changing the 2 instances of UA-XXXXXXXX-X to your own Google Analytics ID) to your site's '_themes/boma/js/custom.js' file.
var gaProperty = 'UA-XXXXXXXX-X';
var disableStr = 'ga-disable-' + gaProperty;
if (document.cookie.indexOf(disableStr + '=true') > -1) {
window[disableStr] = true;
}
function gaOptout() {
document.cookie = disableStr + '=true; expires=Thu, 31 Dec 2099 23:59:59 UTC; path=/';
window[disableStr] = true;
alert('Das Tracking ist jetzt deaktiviert');
}
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
ga('create', 'UA-XXXXXXXX-X', 'auto');
ga('set', 'anonymizeIp', true);
ga('send', 'pageview');
I hope this helps.