MS: Introduce action `before_signup_header`.

This aligns `wp-signup.php` a bit more `with `wp-activate.php` and, among other things, allows a plugin to redirect signup requests.

Props pbearne.
Fixes #17630.


git-svn-id: https://develop.svn.wordpress.org/trunk@35159 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-signup.php

@@ -40,6 +40,13 @@ if ( !is_main_site() ) {
 // Fix for page title
 $wp_query->is_404 = false;
 
+/**
+ * Fires before the Site Signup page is loaded.
+ *
+ * @since 4.4.0
+ */
+do_action( 'before_signup_header' );
+
 /**
  * Prints styles for front-end Multisite signup pages
  *