Multisite: Fire the ms_loaded action after multisite's bootstrap has finished.

This allows code in `sunrise.php` and in core to know when multisite bootstrap is complete and when the `$current_site` and `$current_blog` globals are populated.

Fixes #37235.
Props flixos90.


git-svn-id: https://develop.svn.wordpress.org/trunk@37916 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-settings.php

@@ -104,3 +104,11 @@ if ( ! $current_blog instanceof WP_Site ) {
 
 // Define upload directory constants
 ms_upload_constants();
+
+/**
+ * Fires after the current site and network have been detected and loaded
+ * in multisite's bootstrap.
+ *
+ * @since 4.6.0
+ */
+do_action( 'ms_loaded' );