Save pointers to data in config

[mikefab]

Magic box currently serves

• admin boundary files with population - topojson
• healthsites.io facilities - CSV

When user saves config, the entire config, with data, gets saved on server. Haven't decided what to do with files user drops into kepler, but above data sets shouldn't be copied.

Need to save pointer to dataset, and recreate when config is fetched on front-end.