Allow stuck emails to be manually bypassed

JIRA's mail queue is synchronous, which means that if a mail (or trackback ping) hangs, the whole queue is hung until JIRA is restarted (and the queued items lost). Short of a proper fix (sending mails on a separate thread which can be timed out), we should give admins the ability to 'unstick' the queue, leaving the stuck thread to process its queue item and let the other mails through.