Move new user notification emails to add_action() callbacks.

When a new user is created in various places throughout the interface,
notifications are sent to the site admin and the new user. Previously, these
notifications were fired through direct calls to `wp_new_user_notification()`,
making it difficult to stop or modify the messages.

This changeset introduces a number of new action hooks in place of direct calls
to `wp_new_user_notification()`, and hooks the new wrapper function
`wp_send_new_user_notifications()` to these hooks.

Props dshanske, thomaswm, boonebgorges.
Fixes #33587.

git-svn-id: https://develop.svn.wordpress.org/trunk@34251 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/network/site-users.php

@@ -77,9 +77,16 @@ if ( $action ) {
 				if ( false === $user_id ) {
 		 			$update = 'err_new_dup';
 				} else {
-					wp_new_user_notification( $user_id, null, 'both' );
 					add_user_to_blog( $id, $user_id, $_POST['new_role'] );
 					$update = 'newuser';
+					/**
+					  * Fires after a user has been created via the network site-users.php page.
+					  *
+					  * @since 4.4.0
+					  *
+					  * @param int $user_id ID of the newly created user.
+					  */
+					do_action( 'network_site_users_created_user', $user_id );
 				}
 			}
 			break;