Allow users to save their Flatpak apps and data in Persistent Storage (P202 4.1)
# To Do
* [ ] Write the relevant bits of design doc (part of tails/tails#21590)
* [ ] Figure out what to do with conflicting apps, see https://gitlab.tails.boum.org/tails/tails/-/merge_requests/3009#note_284861
* [ ] Fix feature deactivation
* Fails with `Can't unmount target /home/amnesia/.local/share/flatpak, some process is still using it. Please close all applications that could be accessing the target and try again.` I see `gnome-shell` and `kgx` use `/home/amnesia/.local/share/flatpak/exports/share/icons/hicolor/icon-theme.cache`.
* [ ] Restart `gnome-software.service` on PS feature deactivation too? (see https://gitlab.tails.boum.org/tails/tails/-/merge_requests/3068#note_285983)
# Deliverables
* A Tails prototype with persistent Flatpak support
# Performance indicators
* Persistent installation works across reboots: verified by integration tests
issue