Login and Registration: Send nocache_headers() on Multisite account activation pages.

Props herregroen.
Fixes #43917.
Built from https://develop.svn.wordpress.org/trunk@43065


git-svn-id: http://core.svn.wordpress.org/trunk@42894 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-activate.php

@@ -18,6 +18,8 @@ if ( ! is_multisite() ) {
 	die();
 }
 
+nocache_headers();
+
 if ( is_object( $wp_object_cache ) ) {
 	$wp_object_cache->cache_enabled = false;
 }