Replace `window.prompt()` with custom "add alert to list" dialog.

git-svn-id: https://plugins.svn.wordpress.org/wp-pat-twitter/trunk@927436 b8457f37-d9ea-0310-8a92-e5e31aec5664
When exporting a list, don't export alerts from other lists, too.

git-svn-id: https://plugins.svn.wordpress.org/wp-pat-twitter/trunk@927437 b8457f37-d9ea-0310-8a92-e5e31aec5664