Function migrate_sites_at_start 

pub fn migrate_sites_at_start(path: &Path) -> Result<bool, String>

The sites migration as the window runs it at each start (migrate_sites), while the file still holds a site among its accounts: the file copied aside first (before_sites_path), once only, so that a migration that fails at each start never writes over the first copy, the file as it was. Returns whether any site moved.