Allow resuming a partial upgrade download during a future Tails session
Context: #15875 (comment 20544)
This is about "(S2) I don’t have the time to download everything in 1 session. Maybe I’m traveling (Cris) or maybe I have limited time on Tails (Kim). In this case, I would like to resume my download the next time I start Tails."
To solve it, @sajolida proposed that "Tails Upgrader would offer to do an upgrade again next time, maybe with a slightly adjusted prompt, and then resume the download and the progress bar where it stopped."
This seems doable when the user has a Persistent Storage unlocked. Instead of downloading to /tmp, Tails Upgrader could download to the Persistent Storage.