extract inline JS for google tag manager

app/assets/javascripts/google-tag-manager.js

@@ -0,0 +1,17 @@
+(function() {
+  const gtmDataElement = document.getElementById("data-google-tag-manager");
+  const dataLayerJson = JSON.parse(gtmDataElement.dataset.dataLayer);
+
+  // dataLayer declaration needs to precede the container snippet
+  // https://developers.google.com/tag-manager/devguide#adding-data-layer-variables-to-a-page
+  window.dataLayer = [dataLayerJson];
+
+  /* eslint-disable */
+  // prettier-ignore
+  (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',gtmDataElement.dataset.containerId);
+  /* eslint-enable */
+})();

app/helpers/application_helper.rb

@@ -31,7 +31,7 @@ module ApplicationHelper
     if SiteSetting.ga_universal_auto_link_domains.present?
       result[:allowLinker] = true
     end
-    result.to_json.html_safe
+    result.to_json
   end
 
   def ga_universal_json

app/views/common/_google_tag_manager_head.html.erb

@@ -1,11 +1,5 @@
-<script>
-dataLayer = [<%= google_tag_manager_json %>];
-</script>
+<meta id="data-google-tag-manager"
+  data-data-layer="<%= google_tag_manager_json %>"
+  data-container-id="<%= SiteSetting.gtm_container_id %>" />
 
-<!-- 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','<%= SiteSetting.gtm_container_id %>');</script>
-<!-- End Google Tag Manager -->
+<%= preload_script 'google-tag-manager' %>

config/application.rb

@@ -117,6 +117,8 @@ module Discourse
       plugin-third-party.js
       markdown-it-bundle.js
       service-worker.js
+      google-tag-manager.js
+      google-universal-analytics.js
     }
 
     # Precompile all available locales