Fetch external stylesheets (which aren't from whitelisted font CDNs) to include in amp-custom style

[westonruter]

Currently when coming across a <link rel=stylesheet> we check if it has an href pointing to an allowed font URL and then it gets allowed, but if it has an href that points somewhere else on an external domain then it gets removed:

https://github.com/Automattic/amp-wp/blob/ac53ee79cba5d93ce1a89d37f303f4121e4566fb/includes/sanitizers/class-amp-style-sanitizer.php#L431-L447

https://github.com/Automattic/amp-wp/blob/ac53ee79cba5d93ce1a89d37f303f4121e4566fb/includes/sanitizers/class-amp-style-sanitizer.php#L348-L352

This can result in styles being broken on a site if they depend on the external styles.

I suggest that instead of just removing the style that we instead fetch the contents of the stylesheet and store it in a transient (with cache key being hash of stylesheet URL) which expires according to the Expires time returned in the response or MONTH_IN_SECONDS, whichever is greater. What we must absolutely avoid is the external stylesheet being fetched and refetched with each request.

See #1082 where a workaround prototype plugin has been put in place until 1.0 lands: https://gist.github.com/westonruter/f272303fc4bf2d5d71fe1bd88a5fcee3