Change default value for active_at

We're overriding the default with null often enough that we should
just change the default.

Consequently, no more phantom blank conversations with oneself should
appear after receiving a group update. They were being added to the
inbox because they were incorrectly initialized with an active_at value.

Fixes #281

js/background.js

@@ -69,8 +69,7 @@
                 name: contactDetails.name,
                 id: contactDetails.number,
                 avatar: contactDetails.avatar,
-                type: 'private',
-                active_at: null
+                type: 'private'
             }).save();
         }
 
@@ -82,7 +81,6 @@
                 members: groupDetails.members,
                 avatar: groupDetails.avatar,
                 type: 'group',
-                active_at: null
             }).save();
         }
 

js/models/conversations.js

@@ -27,7 +27,6 @@
       return {
         unreadCount : 0,
         timestamp   : timestamp,
-        active_at   : timestamp
       };
     },
 
@@ -180,7 +179,7 @@
     },
 
     archive: function() {
-        this.unset('active_at');
+        this.set({active_at: null});
     },
 
     destroyMessages: function() {

js/views/new_conversation_view.js

@@ -93,7 +93,6 @@
 
         createConversation: function() {
             var conversation = new Whisper.Conversation({
-                active_at: null,
                 id: this.getRecipients().at(0).id,
                 type: 'private'
             });

js/views/recipients_input_view.js

@@ -132,7 +132,6 @@
             this.new_contact = new Whisper.ConversationListItemView({
                 el: this.$new_contact,
                 model: new Whisper.Conversation({
-                    active_at: null,
                     type: 'private',
                     newContact: true
                 })