fix: live reload port fallback if port is used (#899)

* Move start reload server into separate module

* Find an unused port when starting the live reload server

* Move findUnusedPort into module

* Add tests for findUnusedPort module

* Refactor findUnusedPort

* Move starting of servers into separate module and add tests

* Remove unused constants.js

* Zap extra line breaks

* Add tests for liveReloadServer

* Rename serverController to start

* Move start into lib/server

* Add portfinder package

* Replace findUnusedPort with portfinder

* nits

lib/server/start.js

@@ -0,0 +1,51 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const program = require('commander');
+const openBrowser = require('react-dev-utils/openBrowser');
+const portFinder = require('portfinder');
+const liveReloadServer = require('./liveReloadServer.js');
+const server = require('./server.js');
+
+const CWD = process.cwd();
+
+function startLiveReloadServer() {
+  const promise = portFinder.getPortPromise({port: 35729}).then(port => {
+    liveReloadServer.start(port);
+  });
+  return promise;
+}
+
+function startServer() {
+  const initialServerPort =
+    parseInt(program.port, 10) || process.env.PORT || 3000;
+  const promise = portFinder
+    .getPortPromise({port: initialServerPort})
+    .then(port => {
+      server(port);
+      const {baseUrl} = require(`${CWD}/siteConfig.js`);
+      const serverAddress = `http://localhost:${port}${baseUrl}`;
+      console.log('Docusaurus server started on port %d', port);
+      openBrowser(serverAddress);
+    });
+  return promise;
+}
+
+function startDocusaurus() {
+  if (program.watch) {
+    return startLiveReloadServer()
+      .catch(ex => console.warn(`Failed to start live reload server: ${ex}`))
+      .then(() => startServer());
+  }
+  return startServer();
+}
+
+module.exports = {
+  startDocusaurus,
+  startServer,
+  startLiveReloadServer,
+};