Replace /download with a page with the download extension

https://tails.boum.org/download/ is the link provided in Tails Installer in Debian. Once we have ISO verification extension it should also be the fallback page to download the ISO image (outside of the installation assistant).

Parent Task: #7552 (closed)

Original created by @sajolida on 10366 (Redmine)

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