Turn on update notifications for Ember admin

Issue #3160
- Use notifications API to display available update notification.
- Remove update_notification handlebars helper as now both the
  check for an available update and the notification handling
  is run from the server's admin controller index method.
- Bind the notification's location property to a css class
  for styling.
- Refactor Ember notifications to better handle notification
  objects.  Move responsibility for css class generation onto
  the notification component.
- Refactor gh-notifications component to take a location argument
  that's used to assign a css class and filter notifications.
Merge pull request #3225 from jaswilli/issue-3160-2

Turn on update notifications for Ember admin