message: add assetPrefix back to next.config.js
diff: @@ -2,13 +2,16 @@ const {join} = require('path')
  const withPlugins = require('next-compose-plugins')
  const mdx = require('@zeit/next-mdx')
  +const assetPrefix = process.env.NOW_URL
  +
  module.exports = withPlugins([
  mdx({extension: /\.mdx?$/})
  ], {
  + assetPrefix,
  pageExtensions: ['js', 'jsx', 'md', 'mdx'],
  publicRuntimeConfig: {
  - assetPrefix: process.env.NOW_URL
  + assetPrefix
  },
  webpack(config, {dev}) {
  