Only track usage with GTM when consent given

We now record user cookie preferences in a cookie set by a cookie banner
when a Google Tag Manager tracking ID is set in the config. This ensures
we aren't tracking usage data without user consent when using Google Tag
Manager

Doing the same when using gtag.js (the other approach to tracking data
for Google Analytics 4) would be a breaking change, but it would be
good to start asking consent in both cases eventually

Author: yndajas

lib/source/layouts/_analytics_ga4_gtm.erb

@@ -1,9 +1,18 @@
 <% if config[:tech_docs][:ga4_gtm_tracking_id].is_a?(String) && !config[:tech_docs][:ga4_gtm_tracking_id].empty? %>
   <!-- Google Tag Manager -->
-  <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','<%= config[:tech_docs][:ga4_gtm_tracking_id] %>');</script>
+  <script>
+    const cookies_policy_cookie = document.cookie
+      .split('; ')
+      .find(cookie => cookie.split('=')[0] === 'cookies_policy')
+      ?.split('=')[1]
+
+    if (cookies_policy_cookie && JSON.parse(cookies_policy_cookie).usage) {
+      (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','<%= config[:tech_docs][:ga4_gtm_tracking_id] %>');
+    }
+  </script>
   <!-- End Google Tag Manager -->
 <% end %>