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.html

@@ -1,8 +1,4 @@
-{{- $page := .page -}}
-
-{{- $shouldShowTitle := .shouldShowTitle -}}
-
-{{- $resource := $page.Resources.GetMatch .name -}}
+{{- $resource := .page.Resources.GetMatch .name -}}
 {{- if not $resource }}
   {{ errorf "No resource named '%s'. %s" .name .page.Permalink }}
 {{ end -}}