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/__mocks__/tiny-lr.js

@@ -0,0 +1,12 @@
+/**
+ * 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 tinylrServer = {
+  listen: jest.fn(),
+};
+
+module.exports = () => tinylrServer;