Make CSS output URL respect baseUrl setting

A typo was preventing this from happening: it was always trying
to load CSS from the root domain instead of one relative to the
base path.

server.js

@@ -41,7 +41,7 @@ let options = {
     ],
   },
   extractCSS: true,
-  cssOutputPath: '/css/styles.css',
+  cssOutputPath: 'css/styles.css',
   publicPath: 'public' + (config.basePath || ''),
   compilerConfig: {
       // custom webpack config