Add dead mans switch

[DanielleSucher]

We have a dead mans switch on the server that alerts us if no data points have been received from the pi lately, but nothing to alert us if no sensor readings have been received by the pi from the arduino lately.

Add a failsafe that keeps track of the time the latest sensor reading was received for each sensor, and alerts the server if no new sensor readings have come in the past [period of time - hour or two?].

On charcutio-web, set up an api call that receives this alert and emails box.team.teammates as needed.