Fixed: Delay health check notifications on startup

Closes #1598

(cherry picked from commit 07f0db477a91b39c1f4b884775c08a55ada487cf)

src/NzbDrone.Core/Notifications/NotificationService.cs

@@ -275,6 +275,13 @@ namespace NzbDrone.Core.Notifications
 
         public void Handle(HealthCheckFailedEvent message)
         {
+            // Don't send health check notifications during the start up grace period,
+            // once that duration expires they they'll be retested and fired off if necessary.
+            if (message.IsInStartupGraceperiod)
+            {
+                return;
+            }
+
             foreach (var notification in _notificationFactory.OnHealthIssueEnabled())
             {
                 try