Added experiment for firefox download promo

app/templates/header.js

@@ -0,0 +1,21 @@
+const html = require('choo/html');
+const assets = require('../../common/assets');
+
+module.exports = function(state) {
+  return html`<header class="header">
+  <div class="send-logo">
+    <a href="/">
+      <img src="${assets.get(
+        'send_logo.svg'
+      )}" alt="Send"/><h1 class="site-title">Send</h1>
+    </a>
+    <div class="site-subtitle">
+      <a href="https://testpilot.firefox.com">Firefox Test Pilot</a>
+      <div>${state.translate('siteSubtitle')}</div>
+    </div>
+  </div>
+  <a href="https://qsurvey.mozilla.com/s3/txp-firefox-send" rel="noreferrer noopener" class="feedback" target="_blank">${state.translate(
+    'siteFeedback'
+  )}</a>
+</header>`;
+};