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/small-image.html

@@ -7,23 +7,21 @@
 -}}
 
 <figure class="figure  figure--image  figure--small">
-  <div class="figure__container">
-    {{ if .linksToFullSizeImage -}}
-      <a href="{{ $image.RelPermalink }}">
-    {{- end -}}
-      <img
-        src="{{ (index $resizedImages "2x").RelPermalink }}"
-        srcset="
-          {{ (index $resizedImages "2x").RelPermalink }} 960w,
-          {{ (index $resizedImages "1x").RelPermalink }} 480w,
-        "
-        {{ with $image.Params.alt | default $image.Title }} alt="{{ . }}"{{ end }}
-        {{ with $image.Title }} title="{{ . }}"{{ end }}>
-    {{ if .linksToFullSizeImage -}}
-      </a>
-    {{- end -}}
-    {{ if .shouldShowTitle }}
-      {{ partial "page/figures/caption.html" . }}
-    {{ end }}
-  </div>
+  {{ if .linksToFullSizeImage -}}
+    <a href="{{ $image.RelPermalink }}">
+  {{- end -}}
+    <img
+      src="{{ (index $resizedImages "2x").RelPermalink }}"
+      srcset="
+        {{ (index $resizedImages "2x").RelPermalink }} 960w,
+        {{ (index $resizedImages "1x").RelPermalink }} 480w,
+      "
+      {{ with $image.Params.alt | default $image.Title }} alt="{{ . }}"{{ end }}
+      {{ with $image.Title }} title="{{ . }}"{{ end }}>
+  {{ if .linksToFullSizeImage -}}
+    </a>
+  {{- end -}}
+  {{ if .shouldShowTitle }}
+    {{ partial "page/figures/caption.html" . }}
+  {{ end }}
 </figure>