Use relative path for _next (WATonomous#40)

Author: ben-z

next.config.js

@@ -1,3 +1,5 @@
 module.exports = {
   reactStrictMode: true,
+  // Partially resolves the relative path issue: https://github.com/vercel/next.js/issues/2581#issuecomment-451710217
+  assetPrefix: './',
 };