newrelic_disable_autorum() on ajax and cron

`newrelic_disable_autorum()` on ajax and cron to prevent the NR JS from appearing on the page.

Maybe addresses https://github.com/GigaOM/legacy-pro/issues/3571

Docs at https://docs.newrelic.com/docs/php/php-agent-api :

> `newrelic_disable_autorum (  )`

> Prevents the output filter from attempting to insert the JavaScript for page load timing (sometimes referred to as real user monitoring or RUM) for this current transaction.