CLDC-1620 Add Google Tag Manager scripts to layout (#964)

* CLDC-1620 Add Google Tag Manager scripts to layout

* Update app/views/layouts/application.html.erb

Co-authored-by: James Rose <james@jbpr.net>

* CLDC-1620 Extract GTM IDs to helper

Co-authored-by: James Rose <james@jbpr.net>

app/helpers/analytics_helper.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+module AnalyticsHelper
+  def get_gtm_container
+    # Additional environments tbc
+    "GTM-M6GS7FF"
+  end
+
+  def get_gtm_id
+    # Additional environments tbc
+    "G-1RH26G5KVP"
+  end
+end

app/views/layouts/application.html.erb

@@ -21,6 +21,34 @@
     </script>
     <%= javascript_include_tag "vendor/polyfill-output-value.js" %>
     <%= javascript_include_tag "vendor/outerHTML.js" %>
+
+    <% gtm_container = get_gtm_container %>
+    <% gtm_id = get_gtm_id %>
+
+    <!-- Google Tag Manager (doesn't store personal info until permission given) -->
+    <script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+    new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+    j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+    'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+    })(window,document,'script','dataLayer','<%= gtm_container %>');</script>
+    <!-- End Google Tag Manager -->
+
+    <% if cookies[:accept_analytics_cookies] == "on" %>
+      <script async src="https://www.googletagmanager.com/gtag/js?id=<%= gtm_id %>"></script>
+    <% else %>
+      <script>
+        // For adding the script once the user has given permission
+        window.analyticsScript = "https://www.googletagmanager.com/gtag/js?id=<%= gtm_id %>";
+      </script>
+    <% end %>
+    <script>
+      window.dataLayer = window.dataLayer || [];
+      function gtag(){dataLayer.push(arguments);}
+      gtag('js', new Date());
+
+      gtag('config', '<%= gtm_id %>');
+    </script>
+
     <%= javascript_include_tag "application", defer: true %>
 
     <% if content_for?(:head) %>
@@ -44,6 +72,13 @@
       document.body.className = ((document.body.className) ? document.body.className + " js-enabled" : "js-enabled");
     </script>
 
+    <!-- Google Tag Manager (noscript) -->
+    <% if cookies[:accept_analytics_cookies] %>
+      <noscript>
+        <iframe src="https://www.googletagmanager.com/ns.html?id=<% gtm_container %>" height="0" width="0" style="display:none;visibility:hidden"></iframe>
+      </noscript>
+    <% end %>
+
     <%= govuk_skip_link %>
 
     <%= govuk_header(