Change outage notification frequency

Currently notification emails are sent if a sensor fails to report data for more than 30 minutes, 1 day and 7 days.

Some users have sensors with weak wifi connectivity, where they often may go offline for more than 30 minutes, triggering a notification. (In at least one instance: by the time the user investigated the outage, the sensor was back online.)

**Proposal: change the initial 30 minute notification to 3 hours, to reduce the occurrences of these annoying/spurious outage notifications.**

Alternatively, make the notification schedule user-configurable. (Maybe someday, but I think it'd be better to have done some usability testing before introducing this new feature.)