[A11y] Cart renewal (#5109)

Allow customers to extend their cart reservation up to 11 times the configured reservation time

src/pretix/static/pretixpresale/js/ui/cart.js

@@ -55,6 +55,7 @@ var cart = {
                 pad(diff_minutes.toString(), 2) + ':' + pad(diff_seconds.toString(), 2)
             );
         }
+        $("#cart-extend-button").toggle(diff_minutes < 3);
     },
 
     init: function () {