Add splash screen when booting intermediary Tails

Using manual installation technique (UUI, GNOME Disks, Disk Utility) should be advertised only as a temporary step before installing Tails for real. Even without mentioning persistence, those devices won’t benefit from upgrades.

We propose to add a splash screen when booting Tails from a manually installed USB device that reuses the graphics of the assistant and proposes to go to the next step: use Tails Installer on a second USB stick.

Blueprint: https://tails.boum.org/blueprint/bootstrapping

Related issues

• Related to #15292 (closed)

Original created by @sajolida on 8836 (Redmine)