Design the new download page

1. Design new wireframes
2. Test the wireframes on paper
3. Write the resulting HTML code

Let’s say that this tickets covers everything needed to have a download page that works with the new extension and has enough CSS to be usable.

The integration with the other pages and the replacement of DAVE will happen on #14630 (closed).

The polishing of the CSS will happen on #14921 (closed).

Feature Branch: master

Original created by @sajolida on 14627 (Redmine)

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