Also add a cron job to perform the task on demand
Change the implementation for Ticket::deleteAttachments to call this method
after simply removing all entries from the %ticket_attachment table

This will include other information described in email filters that mark the
email as rejected. For instance, sender-name, message body content, etc. It
also allows for other matching methods (contains, !=, !contains), rather
than the quick banlist method from EmailFilter::isBanned().

Also fix bug in Filter::__construct that prevented any filter from ever
matching