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/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
                 })