update transfer device setup

[KwadroNaut]

Describe the change

It's possible for Tails to remember the secrets to unlock encrypted volumes. Should either instruct on how to do that or point to (for example) password manager.

How will this impact SecureDrop users?

The more steps and passphrases to remember (or copy from vaults) the more people get annoyed. At the moment, when inserting their export or transfer storage, they will always be asked for the password to unlock, even though there's a tickbox in the UX to remember it. Frustrating for users, unexpected behavior that it doesn't get remembered over reboots.

It's possible to remember it, by copying, after unlocking:
/home/amnesia/.gnome2/keyrings/tails.keyring
/live/persistence/TailsData_unlocked/dotfiles/.gnome2/keyring

It can be done for both the SVS and the work stations, for the latter the easiest would be to use the ./securedrop-admin tailsconfig

Additional context

Maybe it's easiest to make it better in Tails, upstream.
I created https://github.com/freedomofpress/securedrop-docs/pull/429/files because I didn't know that /live/persistence/TailsData_unlocked/dotfiles/.gnome2/keyrings was not automatically created.