Allow doing a backup of the Persistent Storage when cloning

We already have a design on the blueprint (https://tails.boum.org/blueprint/backups) so there's not much UX work left to do.

Next steps

• Wait for #17803 (closed): Since we're going to rewrite the persistence setup code into Python, we can probably share this code with Tails Installer, which will need to learn how to create persistence volumes. This saves implementation cost and more importantly, avoids having 2 different pieces of code that do the same thing in theory.

• Discuss this again early April 2021 in the FT + with Fundraising folks.

Timing

• We want to use this as an opportunity to try a crowdfunding (tails/fundraising#17718)

• We might have dev capacity to do this work in 2021 Q4, iff.

  • we're doing good on Sponsor08 deliverables
  • we've made good progress on the Wayland blockers
  • we postpone starting the port to Bullseye

  If so, crowdfunding around April/June 2021, and do the dev work in 2021Q4. Else, crowdfunding in April/June 2022 and do the dev work in 2022Q3.