style: keep articles from being horizontally compressed

In the default style for openring articles are arranged
horizontally.  This is fine with a small number of articles
or a wide screen, but can look squashed when you don't have
those.  Current behavior, on a "Galaxy S5" in Chrome Devtools:

    https://www.jefftk.com/openring-example-nowrap.png

Instead, give each article a minimum width and tell flexbox to
wrap as needed.  To maintain spacing between the articles
without adding spacing around the edges, follow the approach
described in:

    https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Mastering_Wrapping_of_Flex_Items#Creating_gutters_between_items

This gives us new wrapping behavior on narrow screens, without
changing behavior on wider screens:

    https://www.jefftk.com/openring-example-wrap.png