Aspect-preserved detail image with full-size download

Photo detail page previously showed a square crop of the source image. Now
generate an aspect-preserved medium JPEG (longest side 1600, quality 88)
and display that on the detail page instead. The original file is linked
from a click-through on the image and from a sidebar 'Full size' download
link.

Also skip regeneration of images/thumbs/medium when the destination is
already at least as new as the source, so a rebuild triggered by editing
one sidecar (or restarting the service) no longer redecodes and rescales
every photo.

templates/photo.html

@@ -13,7 +13,9 @@
     <a href="/" class="back-btn">← Back</a>
     <div class="dlayout">
       <div class="dimg-wrap">
-        <img src="/{{.Photo.File}}" alt="{{.Photo.Caption}}">
+        <a href="/{{.Photo.File}}" target="_blank" rel="noopener">
+          <img src="/{{.Photo.Medium}}" alt="{{.Photo.Caption}}">
+        </a>
       </div>
       <div class="dsidebar">
         <div class="dauthor">
@@ -22,6 +24,7 @@
         </div>
         <p class="dcaption">{{.Photo.Caption}}</p>
         <p class="ddate">{{.Photo.Date.Format "Jan 02, 2006"}}</p>
+        <p class="dfull"><a href="/{{.Photo.File}}" download>↓ Full size</a></p>
       </div>
     </div>
   </div>