Re-connect saving status URL with Rails app

[jywarren]

After #1126, we need to re-instate the extra method which saves the location of the remote Cloud Exporter status JSON file to the Exports model:

	
          /**	
           * TODO: update API to say if you pass in a custom `handleStatusUrl` you must also build your own updater	
           * and also create your own a frequency	
           * or fix this part	
           */	
          $.ajax({	
            url: "/export",	
            type: 'POST',	
            data: { status_url: statusUrl }	
          }).done(function (data) {	
            console.log('success!! ' + data);	
            updateInterval();	
          });

That sends up here: https://github.com/publiclab/mapknitter/pull/782/files#diff-0d9df75c86ab6387aa553c69b3e0033bR90

And we need to add it back in using the new API for LDI, as we do on these lines:

https://github.com/publiclab/Leaflet.DistortableImage/pull/485/files#diff-885e023f7555a1d19aae1eba08de7d3bR101-R123

Later, we need to display the different status.json file contents in a table in the Exports tab.

[jywarren]

        // initialize the collection:
        imgGroup = L.distortableCollection({
          collection: json, // here we override the image data sent with a custom set
          fetchStatusUrl: fetchStatusUrl,
          handleStatusResponse: handleStatusResponse,
          exportUrl: 'http://34.74.118.242/api/v2/export/', // used to 
          exportStartUrl: 'http://34.74.118.242/api/v2/export/' // used to initiate the export
});