Change talk icon from `comment-o` to `calendar`

Close #527

layouts/partials/talk_li_simple.html

@@ -1,5 +1,5 @@
 <div class="pub-list-item" style="margin-bottom: 1rem" itemscope itemtype="http://schema.org/Event">
-  <i class="fa fa-comment-o pub-icon" aria-hidden="true"></i>
+  <i class="fa fa-calendar pub-icon" aria-hidden="true"></i>
   <span itemprop="name"><a href="{{ .Permalink }}">{{ .Title }}</a></span>
   <div itemprop="startDate">
     {{ $date := .Params.time_start | default .Date }}