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/assets/stylesheets/main.scss

@@ -60,6 +60,7 @@ $path: '/static/images/';
 @import 'components/list-entry';
 @import 'components/letter';
 @import 'components/vendor/breadcrumbs';
+@import 'components/vendor/responsive-embed';
 
 @import 'views/job';
 @import 'views/edit-template';