Higher-res YouTube thumbnails

[twm]

https://tools.simonwillison.net/youtube-thumbnails?url=CRpHNB87gRY shows several higher-res endpoints than the https://i.ytimg.com/vi/CRpHNB87gRY/mqdefault.jpg Yarrharr currently injects.

[twm]

As the current implementation notes the higher-resolution maxresdefault.jpg isn't always available. When it's not the result is a 404.

A few options I see:

1. On the frontend, use an <img onerror> handler to fallback from maxresdefault.jpg to hqdefault.jpg. Probably do this with a web component?
2. Sniff which image is available at sanitization time and embed the correct URL.
3. Rewrite to a proxied URL (provided by Yarrharr) that does the fallback internally.

The first has the additional benefit that it provides an extension point for allowing playback.