Enhancement: Rework notification bell

We're now showing the notification bell based on whether the server supports notifications. Previously it was hidden when there were no notifications.
We've added a badge that displays the amount of new notifications.

https://github.com/owncloud/web/pull/8450
https://github.com/owncloud/web/pull/8454
https://github.com/owncloud/web/issues/8452
