Allow cancellation of newsletter between submission and cron run

After submission of a newsletter, but before the cron run, allow the administrator to change the status of the sending.

For example, switch the locked checkbox and "This newsletter has been sent" with a (defaultly checked) checkbox and "This newsletter is sending". This would change the s_status column from 1 to 0.

I got some help to do this manually through SQL editing by sepeck, but it would be nice to have a UI option.