small tweak to the cover image style
small tweak to the cover image style

this just sets `width: 100%` on the cover image (when we display it on
the post detail page) so that the image looks nice and keeps it's aspect
ratio at all display widths.

pr #1701

mitodl/open-discussions,mitodl/open-discussions,mitodl/open-discussions,mitodl/open-discussions,mitodl/open-discussions