Notifications for Sentinel state change

[Dagnan]

When Sentinel gets out of sync, the UI displays a warning sign. It would make it more useful to also send a notification (similar to e.g. high disk usage) in this case so the problem becomes more visible sooner.

In my case it was due to using the box's public IP and only some of the requests would fail so it took me a while to realize while not being a big issue.