Propose mounting external disks

When an external disk (say, a USB stick) is plugged in, the user should be gently proposed to access it (read: mount it and open a Nautilus window in there).

By default, Gnome mounts external devices ; that’s desactivated in Tails, but a less drastic solution would be better.

Search what Gnome3 proposes in terms of configuration.

Related issues

Is duplicate of #15628 (closed)

Original created by @tails on 5304 (Redmine)

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