Add icons for keychange and expiration timer in-conversation items

The shield matches the Android app's key change notification, and the
clock icon was easy to do and makes it easier to visually distinguish
those items in the conversation history.

FREEBIE

js/views/message_view.js

@@ -76,7 +76,7 @@
 
     Whisper.KeyChangeView = Whisper.View.extend({
         tagName:   'li',
-        className: 'keychange',
+        className: 'keychange advisory',
         templateName: 'keychange',
         id: function() {
             return this.model.id;