feat: add youtube fallback styling

[ShayPunter]

Fixes [#6193]GoogleChrome/developer.chrome.com#6193

Changes proposed in this pull request:

• Implements some fallback styling for web.dev for youtube lite

How to test

1. Clone webdev-infra and checkout branch 6193_youtube_embed_should_show_fallback_link
2. Clone web.dev repository and checkout branch 6193_youtube_embed_should_show_fallback_when_js_is_disabled
3. Setup a sym link between the webdev-infra and web.dev repository
4. Navigate to /advancing-framework-ecosystem-cds-2019/ with javascript enabled, you should see the youtube video as expected
5. Disable javascript
6. Refresh the page, you should now see the fallback text in place of the youtube video

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