Backup should also clear cache browser

When one loads a backup (because it contains more pages, e..g from a dfiferent device), it still fails to open up the latest page. It only works when:

1. one clears the site data in the browser first
2. load the backup
3. then login

This relates to #16. Hopefully the first step can be done automatically when loading your backup. From a user-perspective this is more convenient.

