Add "download" or "export" button to LabelMap 2.0 to download data as currently shown

With LabelMap 2.0, I think we should have a "download" button that allows you to download the currently filtered data (exactly as shown) either as GeoJSON or CSV. This is quite common and I think could be useful for folks (and very fast to implement for us).

(I could be convinced that the "download" button could be "export" instead, not sure).

