text: Title: Support `Last-Modified` header for static files
  Body: Render the `Last-Modified` header when serving static files. We are already performing `.stat()` on the open file handle, so we should have the required modification date.
  
  When serving, also check the [If-Modified-Since](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since) header, and render `HTTP 304 Not Modified` if appropriate.
  
  This issue is very similar to #2243, but probably slightly easier, so maybe this one could be taken first.