Fix url of @import not being rewritten

[pamtbaau]

This is a suggested fix for the issue mentioned in issue #3749

In short:

When using css imports using syntax @import 'mystyle.css'; the url is not being rewritten.

This PR:

• adapts the regex used to include above syntax.
• adapts method Pipeline::cssRewrite() to the newly added matches found by the regex.

Tests

The code has been tested on the following use-cases in /user/themes/quark/css/custom.css:

@import 'import-using-string.css';
@import url('import-using-url.css');

h3 {
    background-image: url('../assets/background-url-quotes.png');
    background-image: url(../assets/background-url-noquotes.png);
}

All urls are correctly rewritten.