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

tests/app/main/views/test_headers.py

@@ -12,4 +12,5 @@ def test_owasp_useful_headers_set(app_):
         "object-src 'self';"
         "font-src 'self' data:;"
         "img-src 'self' *.google-analytics.com *.notifications.service.gov.uk data:;"
+        "frame-src www.youtube.com;"
     )