Subcategories on 0.5.1

Works with https://github.com/NodeBB/NodeBB/commit/f0d3b5dd00b2f813ec673807af9a95b2b22224e6 and  https://github.com/NodeBB/nodebb-theme-vanilla/commit/fcc7a9e098fe13f0ab14b705684e26480a7d707a