Prepare thumbnails of all images in the photo post

Add <img> tags for all thumbnails to the photos list entry. I'm hoping with lazy
loading and some JavaScript this will not be terrible!

Break the logic for preparing a thumbnail into a separate helper partial
template so it can be reused.

layouts/partials/photos/thumbnail_for_list.html

@@ -0,0 +1,28 @@
+{{ $thumbnailResource := .Image }}
+
+{{ $orientation := partial "images/orientation_angle.html" $thumbnailResource }}
+
+{{ $targetWidth := 0 }}
+{{ if isset . "Width" }}
+  {{ $targetWidth = .Width }}
+{{ else }}
+  {{ $targetWidth = $thumbnailResource.Width }}
+{{ end }}
+
+{{ $targetHeight := 0 }}
+{{ if isset . "Height" }}
+  {{ $targetHeight = .Height }}
+{{ else }}
+  {{ $targetHeight = $thumbnailResource.Height }}
+{{ end }}
+
+{{ $thumbnail := false }}
+{{ if not (and (eq $orientation 0)
+               (eq $targetWidth $thumbnailResource.Width)
+               (eq $targetHeight $thumbnailResource.Height)) }}
+  {{ $thumbnail = $thumbnailResource.Fit (printf "%dx%d r%d" $targetWidth $targetHeight (sub 360 $orientation)) }}
+{{ else }}
+  {{ $thumbnail = $thumbnailResource }}
+{{ end }}
+
+{{ return $thumbnail }}

layouts/partials/photos/thumbnails.html

@@ -0,0 +1,51 @@
+{{ $page := .Page }}
+
+{{ $thumbnails := slice }}
+{{ with $thumbnailNamesList := $page.Params "thumbnails" }}
+  {{/* Specify a list of thumbnails to use with the page "thumbnails" param */}}
+  {{ range $thumbnailNamesList }}
+    {{ with $page.Resources.GetMatch . }}
+      {{ $thumbnails = $thumbnails | append . }}
+    {{ else }}
+      {{ errorf "No image resources available for %s from thumbnails param" . }}
+    {{ end }}
+  {{ end }}
+{{ else }}
+  {{/* Get a list of all the image resources and see if any of them specify
+       thumbnails for themselves. If they do, use those, otherwise use the image
+       resource itself. */}}
+  {{ range $img := $page.Resources.ByType "image" }}
+    {{ with $thumbnailResourceName := $img.params.thumbnail }}
+      {{ with $page.Resources.GetMatch $thumbnailResourceName }}
+        {{ $thumbnails = $thumbnails | append . }}
+      {{ else }}
+        {{ errorf "No image resources available for '%s' from thumbnail param for image resources %s"
+            $thumbnailResourceName
+            $img.Name }}
+      {{ end }}
+    {{ else }}
+      {{/* TODO: Look for a named image that indicates it's a thumbnail of 
+           another resources. Something like ${imgBasename}_thumbnail.${ext}.
+           I don't know how easy Hugo makes filename processing, so this could 
+           be painful. */}}
+      {{ $thumbnails = $thumbnails | append $img }}
+    {{ end }}
+  {{ end }}
+{{ end }}
+
+{{ if eq (len $thumbnails) 0 }}
+  {{ errorf "Couldn't find any thumbnails for %s" $page.Permalink }}
+{{ end }}
+
+{{ $thumbnailOptions := dict }}
+{{ if isset . "Width" }}{{ $thumbnailOptions = $thumbnailOptions | merge (dict "Width" .Width) }}{{ end }}
+{{ if isset . "Height" }}{{ $thumbnailOptions = $thumbnailOptions | merge (dict "Height" .Height) }}{{ end }}
+
+{{ $processedThumbnails := slice }}
+{{ range $thumbnails }}
+  {{ $options := $thumbnailOptions | merge "Image" . }}
+  {{ $processedThumbnail := partial "photos/thumbnail_for_list.html" $options }}
+  {{ $processedThumbnails = $processedThumbnails | append $processedThumbnail }}
+{{ end }}
+
+{{ return $processedThumbnails }}