Enable subpages in a namespace

From Bjoern Hassler

Examples:

$wgNamespacesWithSubpages[NS_HELP] = true;
$wgNamespacesWithSubpages[NS_TEMPLATE] = true;
$wgNamespacesWithSubpages[NS_USER] = true;
$wgNamespacesWithSubpages[NS_HELP_TALK] = true;
$wgNamespacesWithSubpages[NS_TEMPLATE_TALK] = true;
$wgNamespacesWithSubpages[NS_USER_TALK] = true;