Fix some corner casese with last seen indicator

* Remove increment behavior
* Dismiss when new messages arrive but the window is focused
* Update the indicator when window becomes focused.

// FREEBIE

js/views/conversation_view.js

@@ -116,7 +116,7 @@
 
             var onFocus = function() {
                 if (this.$el.css('display') !== 'none') {
-                    this.markRead();
+                    this.updateUnread();
                 }
             }.bind(this);
             this.window.addEventListener('focus', onFocus);
@@ -324,11 +324,8 @@
             this.model.messageCollection.add(message, {merge: true});
             message.setToExpire();
 
-            if (this.lastSeenIndicator) {
-                this.lastSeenIndicator.increment(1);
-            }
-
             if (!this.isHidden() && window.isFocused()) {
+                this.removeLastSeenIndicator();
                 this.markRead();
             }
         },

js/views/last_seen_indicator_view.js

@@ -13,11 +13,6 @@
           this.count = options.count || 0;
         },
 
-        increment: function(count) {
-            this.count += count;
-            this.render();
-        },
-
         render_attributes: function() {
             var unreadMessages = this.count === 1 ? i18n('unreadMessage')
               : i18n('unreadMessages', [this.count]);