Design the new download page

Originally created by @sajolida on #14627 (Redmine)

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

Edited May 15, 2020 by sajolida

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