Add kill switch mechanism

This code implements the kill switch mechanism in the extension. This is
explained in //src/killSwitch/README.md and in the design doc:
https://docs.google.com/document/d/1O5YV6_WcxwrUyz-lwHOSTfZ3oyIFWj2EQee0VuKkhaA/edit.

Bug: twpowertools:64

Change-Id: Ia993c78035bba7038aafd53d156f20954217e86f
diff --git a/webpack.config.js b/webpack.config.js
index 615ae14..7d71978 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -1,3 +1,4 @@
+const webpack = require('webpack');
 const path = require('path');
 const json5 = require('json5');
 const CopyWebpackPlugin = require('copy-webpack-plugin');
@@ -85,6 +86,9 @@
           },
         ]
       }),
+      new webpack.DefinePlugin({
+        'PRODUCTION': args.mode == 'production',
+      }),
       ...getCopyPluginsForOverridenLocales(outputPath),
     ],
     // NOTE: Change to