Downloading doesn't show the user when the download is complete

[ellisonbg]

Description

When a user clicks on the button to download the job files, the button goes away, but the links don't automatically show up when the files are downloaded. Instead, the user has to hit reload on the list view to get the updated links. To make matters worse, on the detail view, there is no reload button, so the user has to navigate away and come back (if they figure that out).

Once we can depend on Jupyter Server 2, we should use the new event system to trigger view updates, but in the meantime, we should figure out a way for this to work using polling.

Reproduce

1. Create a job.
2. Wait for it to finish.
3. Click on the download job files button.
4. The links don't show until the user triggers a reload of the view.

Expected behavior

As a user, I want to know that the files are being downloaded and be presented with links when the downloads are ready.

[ellisonbg]

For now it is fine for us to increase the delay from 500ms to 5s to handle this. But it would be great if we can also show an indicator that the files are downloading.