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

Resolves #1508

packages/react-dev-utils/webpackHotDevClient.js

@@ -252,6 +252,10 @@ connection.onmessage = function(e) {
   case 'ok':
     handleSuccess();
     break;
+  case 'content-changed':
+    // Triggered when a file from `contentBase` changed.
+    window.location.reload();
+    break;
   case 'warnings':
     handleWarnings(message.data);
     break;

packages/react-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