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/testutil.tsx

@@ -0,0 +1,20 @@
+// 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 { render } from "@testing-library/react";
+import { SnackbarProvider } from "./snackbars";
+
+export function renderWithCtx(
+  children: React.ReactElement
+): Pick<ReturnType<typeof render>, "rerender"> {
+  function wrapped(children: React.ReactElement): React.ReactElement {
+    return (
+      <SnackbarProvider autoHideDuration={5000}>{children}</SnackbarProvider>
+    );
+  }
+  const { rerender } = render(wrapped(children));
+  return {
+    rerender: (children: React.ReactElement) => rerender(wrapped(children)),
+  };
+}