Upgrade css-loader to 1.0

[stof]

The version of css-loader used by encore currently relies on a buggy version of cssnano to optimize the CSS. Upgrading to the new version should be done: #436 (comment)

[Bonemeijer]

This would also fix the issue we're having where cssnano@3.10.0 (required by css-loader@0.28.11) still relies on an old browserslist version browserslist@1.7.7 and prevents browserslist targeting for iOS