Disable WordPress auto-update

The internal wordpress auto-update mechanism can make it hard for us to control what version of WordPress is running, because if a pod gets auto-updated by its internal wordpress process and then restarted, the old version will be installed again. That could lead to problems, so we should disable the internal process and trust our own Flux updates instead.

To disable this:

define( 'DISALLOW_FILE_EDIT', true ); (this blocks the WP code editors)
define( 'AUTOMATIC_UPDATER_DISABLED', true ); (this blocks minor updates)
define( 'WP_AUTO_UPDATE_CORE', false ); (this blocks the core auto-updates)