Changes to allow active development of UI using webpack and hotloading. (#22)

* Changes to allow active development of UI using webpack and hotloading.

* Update to webpack 4 (will make this work)
* Update webpack.config.js accordingly
* Move webpack.config.js to its own directory
* Split webpack.config.js into base.config.js, dev.config.js and prod.config.js
* Update configs to be "right" for development vs production using --mode

* Want configuration through (optional) local file that is not
  checked in

* Updated package.json for newer babel-loader

* Put in a proxy to localhost port 8080 for evelopment server.

This allows "yarn start" to work on the machine where MoonFire's
server is running. This would be the default situation. Users in
a different setup can change the proxy settings.

webpack/prod.config.js

@@ -0,0 +1,15 @@
+const path = require('path');
+const webpack = require('webpack');
+const merge = require('webpack-merge');
+const baseConfig = require('./base.config.js');
+
+const CleanWebpackPlugin = require('clean-webpack-plugin');
+
+module.exports = merge(baseConfig, {
+  plugins: [
+    new webpack.DefinePlugin({
+      'process.env.NODE_ENV': JSON.stringify('production'),
+    }),
+    new CleanWebpackPlugin(['ui-dist'], { root: path.resolve(__dirname, '../') }),
+  ],
+});