Detect which geojson column (if any) to use for gradient when visualizing user-uploaded layers

[danikam]

The add_uploaded_data_dropdown branch adds a new box called Uploaded Layers with a dropdown from which the user can select geospatial layers that they've uploaded to DataHub for visualization.

We need to add functionality to detect if the user has indicated that they want a particular data column in the geospatial file to be visualized and use that column as gradient. Alternatively, this decision can be made on the spot by providing the user with a list of available data columns.