Update Autoprefixer and standardize browserlist location (#1899)

This updates Autoprefixer to 7.1.1, and also moves the browser target
list to package.json, in line with the recommendation made in this
article (https://evilmartians.com/chronicles/autoprefixer-7-browserslist-2-released)
from the developers of Autoprefixer and Browserslist.

resources/assets/build/webpack.config.optimize.js

@@ -13,7 +13,7 @@ module.exports = {
       cssProcessor: cssnano,
       cssProcessorOptions: {
         discardComments: { removeAll: true },
-        autoprefixer: { browsers: config.browsers },
+        autoprefixer: {},
       },
       canPrint: true,
     }),