This supersedes #20702, but still fixes #18437 (closed).
I found the Jekyll SEO plugin stuff wasn't working right, and having a quick glance at other sites, I realized things were feeling just a tad complicated. Rather than rely on another plugin, this handles it directly in a partial.
I've added description frontmatter for all the pages and included that where appropriate. Also in this PR is two quick images for the Twitter cards (square for most pages, big one for home page).