extend covidcast geo_coverage endpoint to show the inverse

[melange396]

The covidcast geo_coverage endpoint (added in PR #1584) returns the signals that have data for a given geo value (or valueS). The same backing table used there can also be used to return the geo values that have data for a given signal (or signalS). This might be easier to do as a new unique endpoint, or could be added as additional functionality in the existing endpoint.