Server render the main page

Also, add hashes to asset file names and use the "entry manifest" plugin
in dev to get auto-reloading.

modules/actions/serveMainPage.js

@@ -0,0 +1,32 @@
+import React from 'react';
+import ReactDOMServer from 'react-dom/server';
+import { StaticRouter } from 'react-router-dom';
+
+import MainTemplate from '../client/MainTemplate';
+import MainApp from '../client/main/App';
+import createHTML from '../client/utils/createHTML';
+import getEntryPoints from '../utils/getEntryPoints';
+import renderTemplate from '../utils/renderTemplate';
+
+export default function serveMainPage(req, res) {
+  const element = React.createElement(
+    StaticRouter,
+    { location: req.url },
+    React.createElement(MainApp)
+  );
+  const content = createHTML(ReactDOMServer.renderToString(element));
+
+  const entryPoints = getEntryPoints('main', {
+    es: 'module',
+    system: 'nomodule'
+  });
+
+  const html = renderTemplate(MainTemplate, { content, entryPoints });
+
+  res
+    .set({
+      'Cache-Control': 'public, max-age=14400', // 4 hours
+      'Cache-Tag': 'main'
+    })
+    .send(html);
+}