Support pausing and resuming the download of an upgrade
Context: #15875 (comment 20544)
This is above "(S3) The download is eating all my bandwidth and I need to pause it momentarily to do something else. In this case, I would like to be able to pause the download myself and resume it when convenient."
To solve this, @sajolida proposed: "If the user tries to close Tails Upgrader, another dialog could appear and offer to resume the download and the upgrade."
This seems doable when the user has a Persistent Storage unlocked. Instead of downloading to /tmp, Tails Upgrader could download to the Persistent Storage.