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!

layouts/partials/page/figures/fullwidth-image.html

@@ -7,8 +7,12 @@
     "quarter" ($image.Fit "640x640")
 -}}
 
-<figure class="figure  figure--image
-  {{- if eq .size "main" }}figure--main-column{{ end }}">
+{{- $classes := slice "figure" "figure--image" -}}
+{{- if eq .size "main" -}}
+  {{ $classes = $classes | append "figure--main-column" }}
+{{- end -}}
+
+<figure class="{{ delimit $classes "  " }}">
   {{ if .linksToFullSizeImage -}}
     <a href="{{ $image.RelPermalink }}">
   {{- end -}}