restore the threshold widget

[ericnost]

The threshold widget enables filtering the map to show places that exceed various user-defined thresholds for different measures.

It doesn't work entirely because it relies on a defunct layer/table. But it does seem to be returning records? It's currently commented out in index.html

[ericnost]

This is more or less what it should look like: https://storymaps.arcgis.com/stories/0ce18190e6ec4cd3a4843c675233ec9c#ref-n-5ZihVx