Support downloads in the background

[Antolius]

Downloads should happen in the background. User should be free to navigate away from the file that's being downloaded.
The download page should be redesigned to look better, with proper heading and copy explaining the download process (like client certificate picker page does).
User should be able to review all ongoing downloads and stop them.
Notification for successful download should link to directory where the file was saved to.
Errors that might arise during the download should be communicated to the user, with an option to retry the download.