(bug 198: Easy, secure in-place upgrade) Introduce new $wgUpgradeKey. When set in LocalSettings, it allows the user to unlock the installer/upgrader with a hidden key. The days of having to move LocalSettings.php in order to perform an upgrade are gone. The key is the only thing loaded by the installer, you still have to provide the SQL information yourself (as an extra layer of sanity to keep unauthorized users from running it)