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;
 }

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-43063';
+$wp_version = '5.0-alpha-43065';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.