Remove received_at from message detail on outgoing messages

Only show sent_at for outgoing messages, matching Android.

The received_at timestamp reflects the time a message was saved locally.
It is necessary on both incoming and outgoing messages for sorting
purposes, but can be confusing in the context of an outgoing message
detail view, since users don't think about themselves "receiving" their
own messages, and may even interpret this as the time that a message was
received by their conversation partner's device.

// FREEBIE

background.html

@@ -176,7 +176,9 @@
         {{ /hasRetry }}
         <table>
           <tr><td class='label'>Sent</td><td> {{ sent_at }}</td></tr>
-          <tr><td class='label'>Received</td><td> {{ received_at }}</td></tr>
+          {{ #received_at }}
+            <tr><td class='label'>Received</td><td> {{ received_at }}</td></tr>
+          {{ /received_at }}
           {{ #errors }}
             <tr>
               <td class='label'>Error</td>

js/views/message_detail_view.js

@@ -109,7 +109,7 @@
             });
             this.$el.html(Mustache.render(_.result(this, 'template', ''), {
                 sent_at     : moment(this.model.get('sent_at')).toString(),
-                received_at : moment(this.model.get('received_at')).toString(),
+                received_at : this.model.isIncoming() ? moment(this.model.get('received_at')).toString() : null,
                 tofrom      : this.model.isIncoming() ? 'From' : 'To',
                 errors      : this.errors['undefined'],
                 hasRetry    : hasRetry