Fetch CSS over HTTP when URL lacks extension; convert font CDN stylesheets @imports to convert to links instead of fetching

[westonruter]

If there is a stylesheet link with a URL which does not end in css, fetch it over HTTP instead of throwing a validation error.

Additionally, if there is a stylesheet that contains an @import to a stylesheet URL which is whitelisted CDN for fonts such as:

@import "https://fonts.googleapis.com/css?family=Merriweather:300";

Then instead of fetching the CSS to inline, opt instead to convert into a normal link tag:

<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Merriweather:300">

Fixes #1317

[hellofromtonya]

Looks good. Great job @westonruter

[westonruter]

See also Automattic/nginx-http-concat#41 which would bypasse VIP's http-concat for CSS when is_amp_endpoint(). Having this in place will improve the AMP integration yet further, as it would eliminate redundant and unnecessary work.

[kienstra]

Moving To 'Ready For Merging'

If it's alright, I'm moving this to 'Ready For Merging,' as I don't think it needs functional testing. Feel free to move it back.