start a new React-based UI (#111)

This doesn't do much yet but should provide a better foundation for
improvement than the jQuery UI, as described in the github issue.

ui/src/App.test.tsx

@@ -0,0 +1,12 @@
+// This file is part of Moonfire NVR, a security camera network video recorder.
+// Copyright (C) 2021 The Moonfire NVR Authors; see AUTHORS and LICENSE.txt.
+// SPDX-License-Identifier: GPL-v3.0-or-later WITH GPL-3.0-linking-exception
+
+import { screen } from "@testing-library/react";
+import App from "./App";
+import { renderWithCtx } from "./testutil";
+
+test("instantiate", async () => {
+  renderWithCtx(<App />);
+  expect(screen.getByText(/Moonfire NVR/)).toBeInTheDocument();
+});