Fetch stylesheets referenced by @import CSS rules instead of removing them

[westonruter]

When an @import rule is encountered in a stylesheet it is currently getting, it is currently getting deleted because it is not valid AMP:

https://github.com/Automattic/amp-wp/blob/31016b1cc92f8a2e3b75f57dac034b76efafda2e/includes/sanitizers/class-amp-style-sanitizer.php#L692-L697

However, there is no reason why we have to do this (note the @todo's related to this in the style sanitizer). We can instead rather fetch the contents of the @import'ed stylesheet as we're doing for external stylesheets loaded form link tags:

https://github.com/Automattic/amp-wp/blob/31016b1cc92f8a2e3b75f57dac034b76efafda2e/includes/sanitizers/class-amp-style-sanitizer.php#L440-L456

It is import-ant to support @import because it used to be the recommended way for child themes to include the style.css from the parent theme.

Note that we can also support loading @import'ed stylesheets from external domains (and not the filesystem) which will also be done for link tags in #1083.