Put timestamp in a <time> tag for better semantics and easier styling

src/templates/message.html

 <div class="message chat-message {{{o.extra_classes}}}" data-isodate="{{{o.isodate}}}" data-msgid="{{{o.msgid}}}">
-    <span class="chat-msg-author chat-msg-{{{o.sender}}}">{{{o.time}}} {{{o.username}}}:&nbsp;</span>
+    <time timestamp="{{{o.isodate}}}">{{{o.time}}}</time>
+    <span class="chat-msg-author chat-msg-{{{o.sender}}}">{{{o.username}}}:&nbsp;</span>
     <span class="chat-msg-content"><!-- message gets added here via renderMessage --></span>
 </div>