Assign new ID to when importing dashboard via frontend

Fixes a panic: interface conversion: interface is string, not float64
when importing a dashboard that has a non-float ID.

src/app/directives/dashUpload.js

@@ -24,6 +24,7 @@ function (angular, kbn) {
                   return;
                 }
                 var title = kbn.slugifyForUrl(window.grafanaImportDashboard.title);
+                window.grafanaImportDashboard.id = null;
                 $location.path('/dashboard/import/' + title);
               });
             };