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/models/conversations.js

@@ -283,7 +283,7 @@
             var models = this.messageCollection.models;
             this.messageCollection.reset([]);
             _.each(models, function(message) { message.destroy(); });
-            this.save({active_at: null, lastMessage: '', timestamp: null}); // archive
+            this.save({lastMessage: null, timestamp: null}); // archive
         }.bind(this));
     },