need a catchall endpoint

[chriddyp]

Currently, the app only responds when you visit http://localhost:8080. However, the app should respond to any page, for example http://localhost:8080/some-other-page.

Webservers usually have dynamic / wildcard endpoints. We basically want to serve the HTML string for all endpoints except the API endpoints (like /_dash-layout). That way, users can write multi-page apps.