Fix conversation list self-resorting

When deleting all messages in a conversation, the entry in the left pane
should be inserted into the alphabetical portion of the list. To keep it
in this collection, do not nullify active_at.

To ensure the list view is keeping itself correctly sorted, make sure
that resorting behavior is triggered any time a relevant attribute is
changed.

This fixes deleted conversations jumping to the top of the list, and
conversation order scrambling when getting a group or contact sync
message from our master device.

Fixes #734

// FREEBIE

js/conversation_controller.js

@@ -12,7 +12,7 @@
     var conversations = new Whisper.ConversationCollection();
     var inboxCollection = new (Backbone.Collection.extend({
         initialize: function() {
-            this.on('change:active_at', this.sort);
+            this.on('change:timestamp change:name change:number', this.sort);
             this.on('add remove change:unreadCount', this.updateUnreadCount);
 
             this.listenTo(conversations, 'add change:active_at', this.addActive);