M5: notifications, setup check, audit log and password confirmation

- notifications to administrators for failed backup/restore runs and
  locations without a successful backup for two days (once a day)
- setup check in the admin overview: error when the key/kit is not set
  up, warning for missing, old or failed backups
- security events (recovery kit downloaded, location removed,
  append-only turned off, retention changed, snapshots forgotten) go to
  the audit log and are announced to all administrators, from the web
  and from occ; the scheduled weekly cleanup is logged only
- fix: the recovery-kit download link failed the CSRF check (412); it
  now requires a recent password confirmation instead, like creating the
  key, removing a location and restoring

lib/AppInfo/Application.php

@@ -20,6 +20,8 @@ class Application extends App implements IBootstrap {
 	}
 
 	public function register(IRegistrationContext $context): void {
+		$context->registerNotifierService(\OCA\NgBackup\Notification\Notifier::class);
+		$context->registerSetupCheck(\OCA\NgBackup\SetupCheck\BackupStatusCheck::class);
 	}
 
 	public function boot(IBootContext $context): void {