Add `userId` to GA tracking

app/helpers/application_helper.rb

@@ -15,6 +15,15 @@ module ApplicationHelper
   include ConfigurableUrls
   include GlobalPath
 
+  def ga_universal_json
+    cookie_domain = SiteSetting.ga_universal_domain_name.gsub(/^http(s)?:\/\//, '')
+    result = {cookieDomain: cookie_domain}
+    if current_user.present?
+      result[:userId] = current_user.id
+    end
+    result.to_json.html_safe
+  end
+
   def shared_session_key
     if SiteSetting.long_polling_base_url != '/'.freeze && current_user
       sk = "shared_session_key"

app/views/common/_google_universal_analytics.html.erb

@@ -4,7 +4,7 @@
   m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
   })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
 
-  ga('create', '<%= SiteSetting.ga_universal_tracking_code %>', '<%= SiteSetting.ga_universal_domain_name.gsub(/^http(s)?:\/\//, '') %>');
+  ga('create', '<%= SiteSetting.ga_universal_tracking_code %>', <%= ga_universal_json %>);
   ga('send', 'pageview');
 
 </script>