Prevent mass subscription of previously manually unsubscripted email addresses

After a visitor UNsubscribed himself, the admin can still subscribe that person again, (un)intentionally. So, we need an option (enabled by default) which prevents email addresses from being added to mailing lists/ newsletters via mass subscription actions (/admin/content/simplenews/users/import), for email addresses which have been already 'manually' UNsubscribed personally by the owners of those email addresses. This would require that all unsubscribed emailadresses remain stored, which increases the database. Still, aforementioned option would help respecting the opt-out function, which is more user friendly. Nowadays, a (consistently working) opt-out function is required by most anti-spam laws, such as in The Netherlands. This is why, the category of this issue is set to "Bug report" and the status to "Critical".