feat(build): Add source maps to production

We don't have the same precautions as other sites as our code base is
open source, so we might as well make it easier on us to debug
production errors

webpack.config.js

@@ -1,8 +1,9 @@
-var webpack = require('webpack');
-var path = require('path');
-var ManifestPlugin = require('webpack-manifest-plugin');
-var ChunkManifestPlugin = require('chunk-manifest-webpack-plugin');
+const webpack = require('webpack');
+const path = require('path');
+const ManifestPlugin = require('webpack-manifest-plugin');
+const ChunkManifestPlugin = require('chunk-manifest-webpack-plugin');
 const Visualizer = require('webpack-visualizer-plugin');
+const UglifyPlugin = require('uglifyjs-webpack-plugin');
 
 var __DEV__ = process.env.NODE_ENV !== 'production';
 
@@ -10,7 +11,7 @@ module.exports = {
   entry: {
     bundle: './client'
   },
-  devtool: __DEV__ ? 'inline-source-map' : null,
+  devtool: __DEV__ ? 'inline-source-map' : 'source-map',
   node: {
     // Mock Node.js modules that Babel require()s but that we don't
     // particularly care about.
@@ -66,6 +67,11 @@ module.exports = {
 
 if (!__DEV__) {
   module.exports.plugins.push(
+    new UglifyPlugin({
+      test: /\.js($|\?)/i,
+      cache: true,
+      sourceMap: true
+    }),
     new ManifestPlugin({ fileName: 'react-manifest.json' }),
     new ChunkManifestPlugin({
       filename: 'chunk-manifest.json',