Persistence preset: additional software

Right now, the additional software feature exists, but it’s not shown in the persistence configuration. It should be added. When selected, it should open a windows with warnings (cf https://tails.boum.org/doc/advanced_topics/additional_software) and let you enter the list of packages you want.

This is the easier version of #5996 (closed)

Team: alan, sajolida, reviewer (?)

Related issues

Related to #5996 (closed)

Original created by @BitingBird on 9050 (Redmine)

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