Move code for deleting old cookies into banner JS

Removes the following cookies:
- seen_cookie_message (flags if banner was already
  shown)
- _gid (Google Analytics cookie)
- _ga (Google Analytics cookie)

These were set by default before so potentially
still around for some users.

The code for this now exists as a static method on
the cookieMessage module and is called when the JS
loads for the first time.

app/assets/javascripts/cookieMessage.js

@@ -4,6 +4,18 @@ window.GOVUK.Modules = window.GOVUK.Modules || {};
 (function (Modules) {
   function CookieBanner () { }
 
+  CookieBanner.clearOldCookies = function () {
+    // clear any cookies set by the previous version
+    var oldCookies = ['seen_cookie_message', '_ga', '_gid'];
+
+    for (var i = 0; i < oldCookies.length; i++) {
+      if (window.GOVUK.cookie(oldCookies[i])) {
+        var cookieString = oldCookies[i] + '=;expires=' + new Date() + ';domain=' + window.location.hostname.replace(/^www\./, '.') + ';path=/';
+        document.cookie = cookieString;
+      }
+    }
+  };
+
   CookieBanner.prototype.start = function ($module) {
     this.$module = $module[0];
     this.$module.hideCookieMessage = this.hideCookieMessage.bind(this);

app/assets/javascripts/main.js

@@ -1,6 +1,6 @@
 window.GOVUK.Frontend.initAll();
 
-$(() => GOVUK.addCookieMessage());
+window.GOVUK.Modules.CookieBanner.clearOldCookies();
 
 if (window.GOVUK.hasConsentFor('analytics')) {
   window.GOVUK.initAnalytics();

app/templates/admin_template.html

@@ -242,12 +242,4 @@
   <!--[if gt IE 8]><!-->
   <script type="text/javascript" src="{{ asset_url('javascripts/all.js') }}"></script>
   <!--<![endif]-->
-  <script>
-    if (window.GOVUK.cookie('_ga') !== null) {
-      document.cookie = '_ga' + '=;expires=' + new Date() + ';domain=' + window.location.hostname.replace(/^www\./, '.') + ';path=/';
-    }
-    if (window.GOVUK.cookie('_gid') !== null) {
-      document.cookie = '_gid' + '=;expires=' + new Date() + ';domain=' + window.location.hostname.replace(/^www\./, '.') + ';path=/';
-    }
-  </script>
 {% endblock %}