refactor: don't append source map link to built files

My post-receive script doesn't copy sourcemap files to the
`public/static` directory, so it's probably best not to reference them
(browser will try to fetch and print an error in the console).

webpack.production.config.js

@@ -73,6 +73,7 @@ module.exports = BUILDS.map(({name, filename, targets, terserOptions}) => {
         ),
       }),
       new webpack.SourceMapDevToolPlugin({
+        append: false,
         noSources: false,
         filename: '[file].map',
       }),