Allow WP_DEBUG to be disabled via local-config.php

Originally raised in https://github.com/Automattic/vip-quickstart/pull/171 where the objection was raised:

> I don't think we want to be able to disable WP_DEBUG in the dev environment.

However, as the preceding PHP comment “For developers: WordPress debugging mode” instructs, this comment is specifically for developers. And now VIP Quickstart is not only used as a dev environment. When using VIP Quickstart on AWS for a public staging environment, it is not desireable to have these errors displayed as they should instead be written to the `error_log`.