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/photos/rss_item.rss

@@ -1,8 +1,10 @@
-{{- $thumbnail := partial "images/photo_thumbnail.html" (dict "Page" . "Width" 1280 "Height" 1280) -}}
+{{- $thumbnail := partial "photos/thumbnail.html" (dict "Page" . "Width" 1280 "Height" 1280) -}}
 <item>
   {{ partial "rss_item_metadata.rss" . }}
   <description>{{ `<![CDATA[` | safeHTML }}
-    <img src="{{ $thumbnail.Permalink }}">
-    {{ .Content }}
+    {{- range (partial "photos/list.html" .) -}}
+    <img src="{{ .Permalink }}"{{ with .Params.alt }} alt="{{ . }}"{{ end }}>
+    {{- end -}}
+    {{- .Content -}}
   ]]></description>
 </item>