Improve caching of externally-fetched stylesheets

[westonruter]

When AMP_Style_Sanitizer::fetch_external_stylesheet() fails to fetch an external stylesheet, it caches the a WP_Error in the transient as opposed to the stylesheet string. In both cases, the transient expires after a month. This should be more dynamic, namely:

• Fetches that result in failures should be cached for a DAY_IN_SECONDS rather than MONTH_IN_SECONDS.
• Fetches that are successful should actually use a transient expiration that reflects the Cache-Control response header, as long as the max-age is greater than some minimum amount (e.g. 1 day).

See also this todo:

amp-wp/includes/sanitizers/class-amp-style-sanitizer.php

Line 1363 in a3aabc7

* @todo Use Cache-Control max-age for transient.

Originally referenced in in #2449 (comment).