Adapt the UX of Tails Installer to USB images

Now that we are distributing USB images (#15292 (closed)) there are some things that could be adapted in Tails Installer:

USB images are meant for USB sticks but Tails Installer requires an ISO image to “Use a downloaded ISO image”.
Tails Installer is not the canonical way of installing Tails anymore, so the name might be confusing.

Maybe other things.

This ticket is about thinking about the different aspects of this problem.

Original created by @sajolida on 16721 (Redmine)

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