Store the isodate on chat messages in the DOM.

This will be useful later when we want to better place messages in the right
order.

converse.js

@@ -1435,6 +1435,7 @@
                 var message = template({
                     'sender': msg_dict.sender,
                     'time': msg_time.format('hh:mm'),
+                    'isodate': msg_time,
                     'username': username,
                     'message': '',
                     'extra_classes': extra_classes

src/templates/action.html

-<div class="chat-message {{extra_classes}}">
+<div class="chat-message {{extra_classes}}" data-isodate="{{isodate}}">
     <span class="chat-message-{{sender}}">{{time}} **{{username}} </span>
     <span class="chat-message-content">{{message}}</span>
 </div>

src/templates/message.html

-<div class="chat-message {{extra_classes}}">
+<div class="chat-message {{extra_classes}}" data-isodate="{{isodate}}">
     <span class="chat-message-{{sender}}">{{time}} {{username}}:&nbsp;</span>
     <span class="chat-message-content">{{message}}</span>
 </div>