talk: Sort and group talks by time_start instead of date published

* Affects the two types of talk widgets and talk archive
* Requires `time_start` to be present in front matter of all talks

Fix #409

layouts/partials/widgets/talks.html

@@ -19,7 +19,7 @@
   <div class="col-xs-12 col-md-8">
     {{ with $page.Content }}<p>{{ . | markdownify }}</p>{{ end }}
 
-    {{ range first $page.Params.count (where $.Site.RegularPages "Type" "talk") }}
+    {{ range first $page.Params.count (sort (where $.Site.RegularPages "Type" "talk") ".Params.time_start" "desc") }}
       {{ if eq $page.Params.list_format 1 }}
         {{ partial "talk_li_detailed" . }}
       {{ else }}

layouts/partials/widgets/talks_selected.html

@@ -10,7 +10,7 @@
   <div class="col-xs-12 col-md-8">
     {{ with $page.Content }}<p>{{ . | markdownify }}</p>{{ end }}
 
-    {{ range (where (where $.Site.RegularPages "Type" "talk") ".Params.selected" true) }}
+    {{ range (sort (where (where $.Site.RegularPages "Type" "talk") ".Params.selected" true) ".Params.time_start" "desc") }}
       {{ if eq $page.Params.list_format 1 }}
         {{ partial "talk_li_detailed" . }}
       {{ else }}

layouts/section/talk.html

@@ -12,14 +12,14 @@
       <div class="article-style" itemprop="articleBody">{{ . }}</div>
       {{ end }}
 
-      {{ range .Data.Pages.GroupByDate "2006" }}
+      {{ range .Data.Pages.GroupByParamDate "time_start" "2006" }}
       <div class="row" id="talk_list">
         <div class="col-md-2">
           <h3>{{ .Key }}</h3>
         </div>
         <div class="col-md-10">
 
-          {{ range .Pages.ByDate.Reverse }}
+          {{ range sort .Pages ".Params.time_start" "desc" }}
             {{ if eq $.Params.list_format 1 }}
               {{ partial "talk_li_detailed" . }}
             {{ else }}