Private Files: Handle /sites/ paths for Private Files

We’ve added a fix that properly handles files with a subsite path (e.g. /wp-content/uploads/sites/4/2021/01/unpublished.jpg) with our Private Files feature. We now strip the sites/:id portion of the path before forwarding the request to WordPress.

https://github.com/Automattic/vip-go-mu-plugins/pull/1963