Multisite: Introduce ms-site.php and ms-network.php files.
By providing distinct files for the site and network APIs, a better overview is provided. Prior to this change, the ms-blogs.php file had grown too big, mixing site APIs, network APIs and related legacy APIs that need to be maintained. Since multisite is often used in unexpected ways, backward-compatibility is ensured by including the two new files from ms-blogs.php, which previously contained all functions that have been moved to the new files.
This changeset does not contain any functional changes.
Fixes #40647.