This patch implements the changes recommended in issue #610
following the overhaul of the notification API introduced in
Nextcloud 17:
* Update the signature of the INotifier::prepare method.
* Use the new NotificationManager::registerNotifierService
method instead of NotificationManager::registerNotifier
(deprecated).
Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>