adds more information to download data attributes

Adds several data attributes to download links to better enable analytics.

Adds the following:

 - `data-download="trigger"` general selector to help with adding events
 - `data-download-id="id-of-layer"` institutuion-abc123
 - `data-download-type="type of download"` geojson, shapefile, geotiff, harvard-hgl, generated-geotiff, etc