Fix prod css/sass sourcemaps (#1793)

[bentaber]

See Issue #1793

Sass sourcemaps fail to generate in production builds due to cssnano
combined with OptimizeCSSAssetsPlugin. Moving cssnano into
postcss-loader resolves issue.

More detail https://www.webfoobar.com/node/109

Note eval-source-maps still will not work at all for CSS, so
implementers must use Mix.sourceMaps(true, 'source-map') to allow
development builds to successfully generate sourcemaps.

[bentaber]

Note all tests are passing when run locally. I notice last few commits do not pass appveyor ie: d432101

[JackWH]

I can confirm this PR solved the problem for me.

In my case, I was compiling LESS, SCSS, and JS files with Mix. I was trying to compile app.js, app.less, error.less, landing.less, landing.js, and bootstrap.scss.

After dumping the Webpack configuration to the console before Mix ran, it was mis-calculating the entry points as follows:

entry:       {
  '/js/app':     [
    '/Users/Me/Sites/MySite/resources/js/app/app.js',
    '/Users/Me/Sites/MySite/resources/less/app/app.less',
    '/Users/Me/Sites/MySite/resources/less/app/error.less',
    '/Users/Me/Sites/MySite/resources/less/landing/landing.less',
    '/Users/Me/Sites/MySite/resources/scss/bootstrap.scss',
  ],
  '/js/landing': ['/Users/Me/Sites/MySite/resources/js/landing/landing.js'],
}

This was causing only two sourcemaps to be generated, for /js/app and /js/landing, but none corresponding to the CSS files.

Manually patching this PR causes the entry points to calculate correctly — one for each file — and 6 corresponding sourcemaps to be generated, rather than just 2.