API: Switch data source from NYT to JHU for U.S. data

[flaneuse]

Evaluate if the recent changes to how JHU collects/curates U.S. data makes the data more reliable and consistent than the comparable NYT data. If so, refactor the preprocessing code to use only JHU data.

Will require equivalent changes on the front-end to source documents and footers for all data sources.

[gkarthik]

One possibility to explore is reverting back to the time series data in JHU as opposed to the daily reports. The daily reports don't seem to be corrected as often as the time series. This could help address issues outbreak-info/outbreak.info#155 and outbreak-info/outbreak.info#128