Update to use built-in APIs instead of hard-coding query strings

Author: marthacryan

src/plots/plots.js

@@ -215,10 +215,9 @@ plots.sendDataToCloud = function(gd, serverURL) {
     // the chart back to it once Cloud reports that authentication succeeded.
     // Pass the current page's origin as a query string so Cloud knows where to
     // send the CHART_AUTH_SUCCESS message back to.
-    var uploadUrl = serverURL +
-        (serverURL.indexOf('?') === -1 ? '?' : '&') +
-        'origin=' + encodeURIComponent(window.location.origin);
-    var cloudWindow = window.open(uploadUrl, '_blank');
+    var uploadUrl = new URL(serverURL);
+    uploadUrl.searchParams.set('origin', window.location.origin);
+    var cloudWindow = window.open(uploadUrl.href, '_blank');
     if(!cloudWindow) {
         console.error('Unable to open Plotly Cloud (the popup may have been blocked)');
         gd.emit('plotly_exportfail');