Use USB images instead of ISO images in Tails Installer

Right now we have:

( ) Use a downloaded Tails ISO image

Since we distribute USB image, ISO images are only advertized for DVD and virtual machines. To be consistent with both USB images and Tails Installer being for USB sticks only, it should accept USB images instead of ISO images.

It could also accept both USB images and ISO images but it might be more work and not worth it.

Note that this feature of Tails Installer now only makes sense:

• To install (delete all data): as a shortcut to using an USB image in GNOME Disks. Tails Installer is simpler to use than GNOME Disks.
• To upgrade (keep the Persistence): as a shortcut to using an intermediary Tails. I don’t really see this being used to update Tails to it’s latest official version but more to update USB sticks to beta or testing versions.

My guess is that both of these use cases are not so common.

Related issues

• Related to #14544
• Related to #16721 (closed)

Original created by @sajolida on 16906 (Redmine)