Enable watchContentBase for webpack-dev-server (facebook#1546)

Resolves facebook#1508
# Conflicts:
#	packages/react-dev-utils/webpackHotDevClient.js

packages/react-cy-scripts/scripts/start.js

@@ -249,6 +249,8 @@ function runDevServer(host, port, protocol) {
     // for some reason broken when imported through Webpack. If you just want to
     // use an image, put it in `src` and `import` it from JavaScript instead.
     contentBase: paths.appPublic,
+    // By default files from `contentBase` will not trigger a page reload.
+    watchContentBase: true,
     // Enable hot reloading server. It will provide /sockjs-node/ endpoint
     // for the WebpackDevServer client so it can learn when the files were
     // updated. The WebpackDevServer client is included as an entry point