Allow single e-mail address notification to override security.

A lot of users would like to e-mail a particular e-mail address with notifications ignoring security. That is, send e-mails even if anonymous users cannot see the issues. For more background information see the comments on JRA-7402.

At the moment the notification system is written in such a way that it is impossible to develop a Notification Type that e-mails users that are not allowed to see the issue. So to get this to work, we will need to refactor the notification system.

The current work around is to create a user with a particular e-mail address, and ensure that this user has permission to browse the given issues.