Add liveness tracker #5
Description
Make /getLightState/ note the timestamp and save it into the uuid's entry in traffic.json as "heartbeat". This information will tell how how long since the traffic board pinged home. Since the website also calls this function, add a variable that suppresses the recording of this heartbeat.
Provide either this timestamp or a "stale" boolean on calls to /getLightIDs. Controllers that are stalled should be highlighted in light red in the refreshUUIDs function.