Update Google Analytics tracker config

Removes the following fields from the tracker
config:
- `name`, which was erroring due to it
  including a `.`
- `displayFeaturesTask` which seems to be
  deprecated

Also refactors the `create` command to put all
fields into the options parameter, as shown in the
developer docs:

https://developers.google.com/analytics/devguides/collection/analyticsjs/creating-trackers

app/assets/javascripts/analytics/analytics.js

@@ -6,10 +6,14 @@
   // Stripped-down wrapper for Google Analytics, based on:
   // https://github.com/alphagov/static/blob/master/doc/analytics.md
   const Analytics = function (config) {
-    window.ga('create', config.trackingId, config.cookieDomain, config.name, { 'cookieExpires': config.expires * 24 * 60 * 60 });
+    window.ga('create', {
+      'trackingId': config.trackingId,
+      'cookieDomain': config.cookieDomain,
+      'cookieExpires': config.expires * 24 * 60 * 60
+    });
 
     window.ga('set', 'anonymizeIp', config.anonymizeIp);
-    window.ga('set', 'displayFeaturesTask', config.displayFeaturesTask);
+    window.ga('set', 'allowAdFeatures', config.allowAdFeatures);
     window.ga('set', 'transport', config.transport);
 
   };

app/assets/javascripts/analytics/init.js

@@ -24,9 +24,8 @@
         trackingId: trackingId,
         cookieDomain: 'auto',
         anonymizeIp: true,
-        displayFeaturesTask: null,
+        allowAdFeatures: false,
         transport: 'beacon',
-        name: 'GOVUK.analytics',
         expires: 365
       });