Delay namespace initialization

This is needed to allow users to modify $smwgNamespaceIndex, which
is used to determine the value of the namespace constants.

This breaks setting smwgNamespacesWithSemanticLinks directly in LocalSettings.
Instead it will need to be set in a delayed callback, as follows:

$GLOBALS['wgExtensionFunctions'][] = function() {
    // do stuff with $GLOBALS['smwgNamespacesWithSemanticLinks']
};

Change-Id: Ic12b8b391b07aa35fa2ffa1074cf3b67c265e2ba