To add a Subpages link to your Tools in the sidebar,
add this line:
mw.util.addPortletLink ('p-tb', mw.config.get( 'wgServer' ) + mw.config.get( 'wgArticlePath' ).replace("$1", "Special:PrefixIndex/" + mw.config.get( 'wgPageName' )+"/"), 'Subpages');
to your Common.js page and purge per the instructions at the top of the page.