FIX: Always ensure notifications are treated as read once clicked

UX: improve messaging so notifications list is far more stable
PERF: improve performance of notifcation lookup queries

- Add feature "SetTransientHeader" that allows shipping info to server
   in the next Ajax request
- remove local storage hack used for notifications
- amend lookupStale to return hydrated objects, move logic into store
- stop magically clearing various notifications (likes, invitee accepted, group_summary, granted badge)

app/controllers/application_controller.rb

@@ -33,6 +33,7 @@ class ApplicationController < ActionController::Base
   end
 
   before_filter :set_current_user_for_logs
+  before_filter :clear_notifications
   before_filter :set_locale
   before_filter :set_mobile_view
   before_filter :inject_preview_style
@@ -137,6 +138,28 @@ class ApplicationController < ActionController::Base
     response.headers["X-Discourse-Route"] = "#{controller_name}/#{action_name}"
   end
 
+  def clear_notifications
+    if current_user && !Discourse.readonly_mode?
+
+      cookie_notifications = cookies['cn'.freeze]
+      notifications = request.headers['Discourse-Clear-Notifications'.freeze]
+
+      if cookie_notifications
+        if notifications.present?
+          notifications += "," << cookie_notifications
+        else
+          notifications = cookie_notifications
+        end
+      end
+
+      if notifications.present?
+        notification_ids = notifications.split(",").map(&:to_i)
+        Notification.where(user_id: current_user.id, id: notification_ids).update_all(read: true)
+        cookies.delete('cn')
+      end
+    end
+  end
+
   def set_locale
     I18n.locale = current_user.try(:effective_locale) || SiteSetting.default_locale
     I18n.ensure_all_loaded!