Send notification in case of invalid readings

We have seen recently that the sensor can produce invalid (negative) readings under certain extreme conditions (e.g., moisture saturating the mic) See https://dashboard.tracket.info/locations/747559 around January 1st.

We'll be adjusting the sensor to clip values at zero (https://github.com/CivicTechTO/tRacket-sensor/issues/40) but it would be useful to have a notification for users that can warn when such unexpected values occur. We should discuss the exact threshold for the notification (e.g., measuring zero 3 times within a 6 hour period) and what the notification should say.