Add introducer video to the product page

It’s always been the plan to put this video on the product page. Just
getting round to it now.

Watch the video here: https://www.youtube.com/watch?v=i2a2jiwYTd4

Uses the CSS developed by the service manual team, taken from here:
a5c613f07b/source/stylesheets/modules/_responsive-embed.scss

app/templates/views/signedout.html

@@ -105,6 +105,16 @@
       </div>
     </div>
   </div>
+  <div class="product-page-section">
+    <h2 class="with-keyline">
+      Introducing GOV.UK Notify
+    </h2>
+    <div class="responsive-embed responsive-embed--16by9 responsive-embed--bordered bottom-gutter-2">
+      <div class="responsive-embed__wrapper">
+        <iframe width="560" height="315" src="https://www.youtube.com/embed/i2a2jiwYTd4" frameborder="0" allowfullscreen></iframe>
+      </div>
+    </div>
+  </div>
   <div class="product-page-section">
     <div class="with-keyline bottom-gutter-2">
       <h2>Who’s using GOV.UK Notify</h2>