Simplify figure styles and markup

Remove the .figure__container element. According to Tess, putting a <figcaption>
inside a child of a <figure> "doesn't obey the content model" or some shit.
Fiiiiine.

Getting small figures to lay out the way I want them to without the container
was a bit of a trick. Instead of letting the figure span the entire width of the
grid column, let it fit the size of its contents, and then set margin-inline to
auto so it centers.

Doing this also allowed me to remove the .figure--image, etc styles because the
styles and markup are a lot more similar among figures containing different
kinds of elements. Nice!

assets/css/010_spacing.css

@@ -68,11 +68,6 @@ hr {
       "table"
       ".codeblock"
       ".figure"
-      ".figure--main-column"
-      ".figure--code"
-      ".figure--image"
-      ".figure--video"
-      ".figure--youtube"
       ".highlight"
       ".instagram-media"
     )