Multisite: Use get_network() and get_current_network_id() for current network data.

`get_network()` falls back to the current network when called without any arguments. Between this and `get_current_network_id()`, we can replace almost all instances of the global `$current_site` and all instances of `get_current_site()`.

This effectively deprecates `get_current_site()`, something that we'll do in a future ticket.

Props flixos90.
Fixes #37414.

Built from https://develop.svn.wordpress.org/trunk@38814


git-svn-id: http://core.svn.wordpress.org/trunk@38757 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/ms-deprecated.php

@@ -28,7 +28,7 @@ function get_dashboard_blog() {
     if ( $blog = get_site_option( 'dashboard_blog' ) )
         return get_blog_details( $blog );
 
-    return get_blog_details( get_current_site()->blog_id );
+    return get_blog_details( get_network()->site_id );
 }
 
 /**