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.
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