Ensure timer updates show in convo before initiating message

js/models/conversations.js

@@ -878,11 +878,17 @@
       });
 
       source = source || textsecure.storage.user.getNumber();
-      const timestamp = receivedAt || Date.now();
+
+      // When we add a disappearing messages notification to the conversation, we want it
+      //   to be above the message that initiated that change, hence the subtraction.
+      const timestamp = (receivedAt || Date.now()) - 1;
 
       const message = this.messageCollection.add({
+        // Even though this isn't reflected to the user, we want to place the last seen
+        //   indicator above it. We set it to 'unread' to trigger that placement.
+        unread: 1,
         conversationId: this.id,
-        type: receivedAt ? 'incoming' : 'outgoing',
+        // No type; 'incoming' messages are specially treated by conversation.markRead()
         sent_at: timestamp,
         received_at: timestamp,
         flags: textsecure.protobuf.DataMessage.Flags.EXPIRATION_TIMER_UPDATE,