Force-render CSS on basePath

This is a very hacky way of doing it: a base path of /seance will
lead to rendering in,

  public/seance/seance/css/styles.css

but it seems to be the only way out for now 🙁

Hopefully in future we'll come up with a more elegant solution.

server.js

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