Add information about data updates to map

[lossyrob]

There should be clear information on the US Healthcare System Capacity site that shows information about when the data was last updated. As much as possible that makes sense on the UI, we should also try to describe the source of the data and updates in a way that communicates that our current data does not integrate real-time data.

See conversation at https://gitter.im/covid19-healthsystemcapacity/community?at=5e7ba4ad0686790f6185164b