Upgrade to NodeJS 12

Update javascript deps to latest version.
Rebuild assets

- Keep `stylehint` in a lower version because starting at v9.0.0 it generates too much errors. We might fix them later.
- Don't touch `materialize-css` to avoid too much side effect. We might update it later (to 1.0.0).

postcss.config.js

@@ -1,7 +0,0 @@
-module.exports = {
-  plugins: function () {
-    return [
-      require('autoprefixer'),
-    ];
-  }
-};