Make some improvements to the fullwidth-image partial

- Add some handling of SVGs so the template doesn't attempt to resize them.
- Include custom classes in the <figure class> attribute
- Do not link to SVG images
- Only include <img srcset> if there are multiple sized images. This mostly supports
  SVG figures where there's only ever one.

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

@@ -1,28 +1,39 @@
 {{- $image := .resource -}}
 
-{{-
-  $resizedImages := dict
-    "full"    ($image.Fit "2560x2560")
-    "half"    ($image.Fit "1280x1280")
-    "quarter" ($image.Fit "640x640")
--}}
+{{- $isSVG := strings.Contains $image.MediaType.SubType "svg" -}}
+{{- $linksToFullSizeImage := and .linksToFullSizeImage (not $isSVG) -}}
+
+{{- $resizedImages := dict "full" $image -}}
+{{- if not $isSVG -}}
+  {{-
+    $resizedImages := dict
+      "full"    ($image.Fit "2560x2560")
+      "half"    ($image.Fit "1280x1280")
+      "quarter" ($image.Fit "640x640")
+  -}}
+{{ end }}
 
 {{- $classes := slice "figure" "figure--image" -}}
 {{- if eq .size "main" -}}
   {{ $classes = $classes | append "figure--main-column" }}
 {{- end -}}
+{{- with .class -}}
+  {{ $classes = $classes | append . }}
+{{- end -}}
 
 <figure class="{{ delimit $classes "  " }}">
-  {{ if .linksToFullSizeImage -}}
+  {{ if $linksToFullSizeImage -}}
     <a href="{{ $image.RelPermalink }}">
   {{- end -}}
     <img
       src="{{ (index $resizedImages "full").RelPermalink }}"
-      srcset="
-        {{ (index $resizedImages "full").RelPermalink }} 2560w,
-        {{ (index $resizedImages "half").RelPermalink }} 1280w,
-        {{ (index $resizedImages "quarter").RelPermalink }} 640w,
-      "
+      {{ if gt (len $resizedImages) 1 -}}
+        srcset="
+          {{ (index $resizedImages "full").RelPermalink }} 2560w,
+          {{ (index $resizedImages "half").RelPermalink }} 1280w,
+          {{ (index $resizedImages "quarter").RelPermalink }} 640w,
+        "
+      {{ end }}
       {{ with $image.Params.alt | default $image.Title }} alt="{{ . }}"{{ end }}
       {{ with $image.Title }} title="{{ . }}"{{ end }}>
   {{ if .linksToFullSizeImage -}}