Add Content-Security-Policy header

Now dev testing can reproduce the security issues we saw in production.

build/webpack.dev.conf.js

@@ -46,6 +46,9 @@ const devWebpackConfig = merge(baseWebpackConfig, {
     quiet: true, // necessary for FriendlyErrorsPlugin
     watchOptions: {
       poll: config.dev.poll
+    },
+    headers: {
+      'content-security-policy': "base-uri 'self'; frame-ancestors 'none'; img-src 'self' data: https:; media-src 'self' https:; style-src 'self' 'unsafe-inline'; font-src 'self'; manifest-src 'self'; script-src 'self';"
     }
   },
   plugins: [