Add a little JS module to track events

Google analytics lets you send:
- pageviews
- events

Page views are used by default. But sometimes you wanna count something
which isn’t the user navigating to a new page, or you wanna track
something which isn’t just what page they were looking it. This is where
events are useful.

This commit adds a small JS module that lets us fire off an event when
the presence of an element with the right data attributes are present on
the page.

app/assets/javascripts/errorTracking.js

@@ -0,0 +1,22 @@
+(function(Modules) {
+  "use strict";
+
+  Modules.TrackEvent = function() {
+
+    this.start = function(component) {
+
+      if (!ga) return;
+
+      ga(
+        'send',
+        'event',
+        'Error',
+        $(component).data('error-type'),
+        $(component).data('error-label')
+      );
+
+    };
+
+  };
+
+})(window.GOVUK.Modules);