[Tails-dev] [de][website] (review) CSS

Hello,

there was a slight problem with the CSS file on the contribute page.
which i'd fix like this:

[de/css bb34145]

* no fixed height for contribute boxes as this does not work in german
anymore.

* align images of the boxes in the middle and add a max-width so if i
have a smaller window, the images will not overlay each other or disappear.

feel free to review and pull at least the first modification. the second
one might need more cosmetics (i know you are very attached to
alignments :p) although i personally find it quite ok like this. I'd
rather rework the box title's appearance than to have strange margins.

cheers,
u.