Enable cheap source-map by default (#977)

Debugging issues in the dashboard is a little frustrating without source maps and the full source map takes awhile to build.

Just enables one of the cheaper source maps by default. It is good enough (tm) for what is there now.

web/app/webpack.config.js

@@ -1,3 +1,5 @@
+/* global require, module, __dirname */
+
 const path = require('path');
 
 module.exports = {
@@ -7,7 +9,7 @@ module.exports = {
     publicPath: 'dist/',
     filename: 'index_bundle.js'
   },
-  // devtool: 'source-map', // uncomment for nicer logging, makes dev slower
+  devtool: 'cheap-module-source-map',
   externals: {
     cheerio: 'window',
     'react/addons': 'react',