Type of work:
- when the notification fails, logging stdout/stderr/returncode of the tails-notify-user
- filter out empty lines from the conf file: else, APT is called in a way that makes it crash
- explain the user that most APT lists expire after a week or so, and that will make offline packages installation fail after a while
- move the args we always want to pass to apt-get (
- verify the need of a custom apt-get PATH and add a comment to explain if it's really needed.
feature/remember_installed_packages, for Tails 0.20.