Improve some terminology and phrasing of the Persistence wizard

• Apply style guide: Persistent Storage
• Use “Assistant” instead of wizard
• Rename the menu entry as “Persistent Storage Settings” or “Persistent Storage” (only)
• While we’re breaking translation let’s improve a bunch of other things as well :)
• Adapt the documentation accordingly
• Rename the “Persistent data” feature as “Persistent folder”
• Improve the error message “Error, Persistence partition is not unlocked.” to get rid of the note on doc/first_steps/persistence/configure.mdwn
• Writes “features” somewhere in the list of features
• Improve strings on Welcome Screen
  • “Persistent Storage” or leave it like this
  • “Unlock Encryption”
• Improve the warning when running from an ISO image (#18045 (closed))

This is on our 2020-2022 roadmap.