Last seen indicator: dismissed only on open and send

It can be moved if you're not scrolled to the bottom of of the window
or the window doesn't have focus when a new message comes in. Other than
that, it marches up the window until you close and reopen the
conversation, or send a message.

Note that we do NOT mark messages as read if they come in when you are
scrolled up. But we do mark the entire conversation as read if you
switch away from the app and back.

FREEBIE

js/views/last_seen_indicator_view.js

@@ -13,14 +13,6 @@
         initialize: function(options) {
           options = options || {};
           this.count = options.count || 0;
-          this.start = Date.now();
-        },
-
-        isOldEnough: function() {
-          var now = Date.now();
-          if (now - this.start > FIVE_SECONDS) {
-            return true;
-          }
         },
 
         increment: function(count) {