Text Changes: Replace "Webmaster" in output messages with more appropriate terms:

* "All at `###SITENAME###`" in `wp-admin/ms-delete-site.php`, for consistency with other Multisite emails.
* "Site admin" in `register_new_user()`, for clarity.

Props Presskopp, marybaum.
Fixes #50394.
Built from https://develop.svn.wordpress.org/trunk@48049


git-svn-id: https://core.svn.wordpress.org/trunk@47816 1a063a9b-81f0-0310-95a4-ce76da25c4cd