Playground: when you import from a URL, relative URLs break

For example, if I import https://camp.samples.amp.dev into the playground, images break, because they're expressed as relative URLs, not absolute URLs.

I assume that, to fix this, you'd have to inject a `` tag into the ``, rewrite URLs, or do something else that might break some pages... but if a clever fix is possible, it could open up new possibilities for the playground as a debugging tool!