Display progress of artifact download

Problem/Motivation

The trial just shows "Downloading artifact", but it's not clear how long it may take. We can display a progress bar of the fetch request.

Instead of using response.json() we'll have to use the body reader directly.

Examples https://github.com/anthumchris/fetch-progress-indicators https://github.com/anthumchris/fetch-progress-indicators/blob/master/fet...

And to note for gzip:

  const contentEncoding = response.headers.get('content-encoding');
  const contentLength = response.headers.get(contentEncoding ? 'x-file-size' : 'content-length');