Improve some terminology and phrasing of the Persistence wizard
Type of work:
User interface design
- From the discussion we had on terminology, we should replace occurrences of "persistent volume" as "persistent storage". See https://mailman.boum.org/pipermail/tails-ux/2015-May/000402.html
- Use "Assistant" instead of wizard: tracked elsewhere (#15109)
- Rename the menu entry as "Persistent Storage Settings".
- While we're breaking translation let's improve a bunch of other things as well :)
- Adapt the documentation accordingly.