Wrap analytics code in GOVUK interface

Wraps our analytics code in a stripped down
version of GOVUK.Analytics to allow us to plug in
the GOVUK code for consent.

app/assets/javascripts/main.js

@@ -2,6 +2,8 @@ window.GOVUK.Frontend.initAll();
 
 $(() => GOVUK.addCookieMessage());
 
+window.GOVUK.initAnalytics();
+
 $(() => $("time.timeago").timeago());
 
 $(() => GOVUK.stickAtTopWhenScrolling.init());