Add Change Healthcare signals to the map

[tildechris]

See https://cmu-delphi.github.io/delphi-epidata/api/covidcast-signals/chng.html

The current signals are based on confirmed-COVID cases, not CLI like the current DV signal.

We need to wait on the new sensorized CLI signal before incorporating this change, so this is a placeholder for now.

[tildechris]

Blocked by cmu-delphi/covidcast-indicators#568

[tildechris]

Closing for now. Let's readdress when the issues with the chng signal are addressed.