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-includes/user-functions.php

@@ -2023,11 +2023,31 @@ function register_new_user( $user_login, $user_email ) {
 
 	update_user_option( $user_id, 'default_password_nag', true, true ); //Set up the Password change nag.
 
-	wp_new_user_notification( $user_id, null, 'both' );
+	/**
+	 * Fires after a new user registration has been recorded.
+	 *
+	 * @since 4.4.0
+	 *
+	 * @param int $user_id ID of the newly registered user.
+	 */
+	do_action( 'register_new_user', $user_id );
 
 	return $user_id;
 }
 
+/**
+ * Initiate email notifications related to the creation of new users.
+ *
+ * Notifications are sent both to the site admin and to the newly created user.
+ *
+ * @since 4.4.0
+ *
+ * @param int $user_id ID of the newly created user.
+ */
+function wp_send_new_user_notifications( $user_id ) {
+	wp_new_user_notification( $user_id, null, 'both' );
+}
+
 /**
  * Retrieve the current session token from the logged_in cookie.
  *