Tweak loading for URL and open data options to indicate loading without progress

[stuartlynn]

Right now we cant give progress updates on URL or Open Data sources. This is because we can't do a range request against these resources which is what papa-parse uses to stream data. Either need to implement this streaming on the proxy server somehow or figure out some other solution but for now we can caution that a dataset might take a while to read and display a spinner rather than a progress bar