Include all photos as links in the RSS and Atom feeds, instead of just the thumbnail

- Move photo_thumbnail.html to photos/thumbnail.html
- Add a photos/list.html partial that returns a list of photos for the current
  page
- Use the template above everywhere we need a list of photos for the page

layouts/shortcodes/photo.html

@@ -1,5 +1,5 @@
 {{- with $photoPage := $.Page.GetPage (printf "photos/%s" (.Get 0)) -}}
-  {{- $thumbnail := partial "images/photo_thumbnail.html" (dict "Page" . "Width" 1280 "Height" 1280) -}}
+  {{- $thumbnail := partial "photos/thumbnail.html" (dict "Page" . "Width" 1280 "Height" 1280) -}}
   {{- $altText := $thumbnail.Params.alt | default .Title -}}
   <a href="{{ .RelPermalink }}" title="{{ .Title }}">
     <img src="{{ $thumbnail.RelPermalink }}"{{ with $altText }} alt="{{ . }}"{{ end }}>