Add an optional photo field to entries, which becomes a card-topping u-photo when displayed

templates/mf2/h-entry.hamlet

@@ -1,4 +1,6 @@
 <article .h-entry .card.bg-dark>
+  $maybe photo <- entryPhoto entry
+    <img .card-img-top.u-photo src=@{staticR ["uploads", photo]} alt=#{entryTitle entry}>
   <div .card-body>
     $maybe name <- entryName entry
       <h4 .p-name .card-title>#{name}