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.js

@@ -141,7 +141,7 @@ let webpackConfig = {
         output: { path: config.paths.dist },
         context: config.paths.assets,
         postcss: [
-          autoprefixer({ browsers: config.browsers }),
+          autoprefixer(),
         ],
       },
     }),