Import: Fix plugin activation link after installing an importer on multisite.

When installing the importer via Ajax, the activation link should be the same as if the page were reloaded.

Props joelcj91 for initial patch.
Fixes #37943.
Built from https://develop.svn.wordpress.org/trunk@38704


git-svn-id: http://core.svn.wordpress.org/trunk@38647 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/updates.js

@@ -1876,6 +1876,7 @@
 
 			wp.updates.installPlugin( {
 				slug:    $button.data( 'slug' ),
+				pagenow: pagenow,
 				success: wp.updates.installImporterSuccess,
 				error:   wp.updates.installImporterError
 			} );