Support multiple persistent paths per preset in the persistent volume assistant

It could be useful to have a single preset in tails-persistence-setup imply adding more than 1 line in persistence.conf.

E.g. the APT lists and packages could be merged to make things easier.

This needs a non-trivial redesign of some assumptions made in t-p-s.

Feature Branch: perl5lib:feature/14594-asp-gui, iuk:feature/14594-asp-gui, persistence-setup:feature/14594-asp-gui, feature/14594-asp-gui

Original created by @intrigeri on 6485 (Redmine)

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