soon be used by admins only to grant swappability. For now, it is open to
anyone who has expt modify access as well.

The change is visible everywhere SHOWEXP is called, like showexp.php3 for
instance.

Also introduced toggle.php, a generic toggle script, similar to
adminmode.php, but useful for just about anything. To add something, just
add it to the list, specify the valid values it can have, and add its
permission check and action. Now you don't need a different php page for
each toggle switch. This page is already set up to take over from
adminmode.php, but I didn't change things over just yet.

(Should this go in the changelog?)