Progressively enhance the proposition illustration

When users with Javascript enabled request it we can show a higher
quality SVG image which will look better in certain circumstances.

app/assets/javascripts/homepage.js

@@ -0,0 +1,24 @@
+(function(window) {
+  "use strict";
+
+  window.GOVUK.Modules.Homepage = function() {
+
+    this.start = function(component) {
+
+      let $component = $(component);
+      let iterations = 0;
+      let timeout = null;
+
+      $component.on('click', () => {
+        if (++iterations == 5) {
+          $component.toggleClass('product-page-intro-wrapper--alternative');
+        }
+        clearTimeout(timeout);
+        timeout = setTimeout(() => iterations = 0, 1500);
+      });
+
+    };
+
+  };
+
+})(window);