Support HTTP caching for static assets

[wwwillchen]

For static assets like HTML+CSS+JS, we should cache them, particularly in prod mode. For editor mode, we could cache them, except as framework developers, it's convenient to not cache them (so maybe a flag to disable caching).

We could try to cache the /ui POST requests because that is much trickier because it would break in many use cases (e.g. clicking a button to fire an API call).