Internal: Add WPCOM_VIP_SITE_ADMIN_ONLY_MAINTENANCE constant

Added a constant, WPCOM_VIP_SITE_ADMIN_ONLY_MAINTENANCE, that can be used to put a site into maintenance mode without impacting the front-end. The idea being to prevent database writes(e.g.: post creation) in wp-admin from happening at an inopportune time during periods of maintenance while still allowing the site to operate normally.

https://github.com/Automattic/vip-go-mu-plugins/pull/2152