Site health: Load the "Info" tab immediately and notify the user while gathering site data. Changes the Info tab to work similarly to the Status tab: it does separate request to fetch the directories sizes and doesn't "block" the loading of the page.

Props xkon, afercia, Clorith, azaozz.
See #46707.

git-svn-id: https://develop.svn.wordpress.org/trunk@45176 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/admin-ajax.php

@@ -136,6 +136,7 @@ $core_actions_post = array(
 	'health-check-is-in-debug-mode',
 	'health-check-background-updates',
 	'health-check-loopback-requests',
+	'health-check-get-sizes',
 );
 
 // Deprecated