FEATURE: cross-domain tracking for Google universal analytics

app/helpers/application_helper.rb

@@ -23,6 +23,9 @@ module ApplicationHelper
     if current_user.present?
       result[:userId] = current_user.id
     end
+    if SiteSetting.ga_universal_auto_link_domains.present?
+      result[:allowLinker] = true
+    end
     result.to_json.html_safe
   end
 

app/views/common/_google_universal_analytics.html.erb

@@ -5,4 +5,10 @@
   })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
 
   ga('create', '<%= SiteSetting.ga_universal_tracking_code %>', <%= ga_universal_json %>);
+
+  <% if SiteSetting.ga_universal_auto_link_domains.present? %>
+  ga('require', 'linker');
+  ga('linker:autoLink', <%= raw SiteSetting.ga_universal_auto_link_domains.split('|').to_json %>);
+  <% end %>
+
 </script>

config/locales/server.en.yml

@@ -1035,6 +1035,7 @@ en:
     ga_domain_name: "OBSOLETE: Google analytics (ga.js) domain name, eg: mysite.com; see http://google.com/analytics"
     ga_universal_tracking_code: "Google Universal Analytics (analytics.js) tracking code code, eg: UA-12345678-9; see http://google.com/analytics"
     ga_universal_domain_name: "Google Universal Analytics (analytics.js) domain name, eg: mysite.com; see http://google.com/analytics"
+    ga_universal_auto_link_domains: "Enable Google Universal Analytics (analytics.js) cross-domain tracking. Outgoing links to these domains will have the client id added to them. See Google's Cross-Domain Tracking guide."
     gtm_container_id: "Google Tag Manager container id. eg: GTM-ABCDEF"
     enable_escaped_fragments: "Fall back to Google's Ajax-Crawling API if no webcrawler is detected. See https://developers.google.com/webmasters/ajax-crawling/docs/learn-more"
     allow_moderators_to_create_categories: "Allow moderators to create new categories"

config/site_settings.yml

@@ -103,6 +103,9 @@ basic:
   ga_universal_domain_name:
     client: true
     default: 'auto'
+  ga_universal_auto_link_domains:
+    default: ''
+    type: list
   ga_tracking_code:
     client: true
     default: ''