css: remove top padding for images

With the removal of side padding, this gives an odd look to images. We
keep the bottom padding as it is needed for multiline titles.
css: code blocks should be wide, even inside lists

We don't handle the nested case. It should be good enough for what we
want.
css: restrict the padding added to `li` to the footer
article: update serverspec article with a note about host classification

Make the reader notice that `spec_helper.rb` should be altered too.