Make dropdown components "multi-select"

[newswim]

Idea: expand the filters for the big Evidence of Representation visualization.

Right now the select dropdowns are all single select (e.g. you can only select Charge Category: “Criminal Traffic”, you can’t say “Criminal Traffic AND Drug”). We can do this through our query interface. The next step would just be to make the select widget a multi-select widget.

This means that our query builder will also need to support this feature.