Progress bar/message on fetch

[changyang-liu]

Some packages take very long to fetch, making it seem like the fetch has failed even though it's still going on. Add a progress bar, or maybe something that indicates the progress of the fetch (e.g. fetching image 1 of 4) on the frontend so that the user knows not to refresh the page.

In the same vein as #82 (issues with slow/failing fetches)