interface: version JS changes in webpack

Also fixes a longstanding bug where the compiled JS would be written
twice to the same file, causing the bundle to be twice as large as it
needs to be.

pkg/interface/config/webpack.prod.js

@@ -56,8 +56,7 @@ module.exports = {
     // }),
   ],
   output: {
-    filename: 'index.js',
-    chunkFilename: 'index.js',
+    filename: 'index.[contenthash].js',
     path: path.resolve(__dirname, '../../arvo/app/landscape/js'),
     publicPath: '/'
   },