Update copying of persistent volume for Jessie

Tails 2.0 comes with GNOME Disks as disk-management tool. The GUI changed a lot, the instructions for copying persistent data to another device need to be updated.

Feature Branch: doc/10936-copy-persistence

Parent Task: #8043 (closed)

Original created by @spriver on 10936 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information