Only use safe options when packing CSS assets (facebook#4706)

* support scoped packages for cra --scripts-version option

* enable safe option to cssnano processor

Fixes facebook#4682

Author: Jack Zhao

config/webpack.config.prod.js

@@ -153,7 +153,7 @@ module.exports = {
         cache: true,
         sourceMap: shouldUseSourceMap,
       }),
-      new OptimizeCSSAssetsPlugin(),
+      new OptimizeCSSAssetsPlugin({ cssProcessorOptions: { safe: true } }),
     ],
     // Automatically split vendor and commons
     // https://twitter.com/wSokra/status/969633336732905474