Fix 304 response for static files (Fixes intermittent blank pages for requests to '/')

- Use native Sinatra static file serving
- Simplify matcher for routing root path to index.html

Sinatra has built-in static file serving that was already serving all static files for the app except `index.html` when accessed through the `/` path. Upon a GET to `/`, the app was formerly responding with the `send_file` method with an overrided `status` of `200`. `send_file`, however sent an empty response body if the file had not been modified since the browser's cached version. In Chrome this was causing every other request to `/` to display a blank page.