css: fix jumbo article header behavior on very narrow / very wide screens.

css: fix jumbo article header behavior on very narrow / very wide screens.

In some cases the content after got floated right after the article
title, on iOS Safari the content was even overlapping the header. Also
added a minimal font size so the (sub)title and date/author text don't
get scaled below 1rem.

Now the cover image also slides a bit under the summary / content, like
it is on landing pages.