Prevent uploaded layers from being clobbered when public layers are applied

[danikam]

Currently on the add_uploaded_data_dropdown branch, uploaded layers selected with the "Uploaded Layers" dropdown get removed from the map when the user selects one or more layers from the right-hand menu and clicks "Apply". Ideally, this action should just add the layers selected from the right-hand menu to the list of layers being visualized on the map.