[Tails-ux] Improve Tails Installer UX

Hi!

Regarding https://labs.riseup.net/code/issues/9005 and several related
tickets: #8861, #8860, #8859, #9006, I think its time to ask for
feedback even if I know there still things to do (Like deleting
*-launcher stuff). But anyway, it would be nice to add changes and fixes
before it gets finished.

Thanks,
kurono