Add single-day pages linked from timeline date headings

Each timeline date heading now links to /day/<YYYY-MM-DD>/, a page
holding just that day's photos with a back link to the timeline.
renderDayPages writes one per groupByDay run; the ISO label doubles
as the URL slug.

templates/timeline.html

@@ -37,7 +37,7 @@
     <div class="grid-wrap">
       {{range .Groups}}
       <section class="tl-day">
-        <h2 class="tl-date">{{.Label}}<span class="tl-count">{{.Count}}</span></h2>
+        <h2 class="tl-date"><a href="/day/{{.Label}}/">{{.Label}}</a><span class="tl-count">{{.Count}}</span></h2>
         <div class="pgrid">
           {{range .Photos}}
           <a href="/photo/{{.Slug}}/" class="ptile">