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/views/inbox_view.js

@@ -68,8 +68,8 @@
             }).render();
 
             this.inboxListView.listenTo(inboxCollection,
-                    'add change:active_at',
-                    this.inboxListView.onChangeActiveAt);
+                    'add change:timestamp change:name change:number',
+                    this.inboxListView.sort);
 
             this.searchView = new Whisper.ConversationSearchView({
                 el    : this.$('.search-results'),