Notifications are generally annoying and not useful

[alexmurray]

trafficstars

Currently notifications are shown when an error occurs reading from a sensor or when a sensor alarms. But in both cases there is no real action the user can take. We should not use notifications in general and be smarter - ie. Show an 😵 or similar on sensor error for the value and perhaps one notification saying that the error occurred, which we then hide if the error stops.

Also since no action can be taken for alarm conditions, don't show a notification for these but instead change the icon to indicate the alarm is active.