Contribute - Add an onscreen indicator that the export is pending 'export'

When running a query and attempting to export the results, there is no ''on screen'' indicator that the report is downloading/processing request.

The export page is displayed and it is not clear if Export was clicked on or not.

This becomes confusing when the downloads takes some time, thinking the export button may not have been clicked, end users have a tendency to re-click the export button which eventually results in error messages