Truly confidential issues

Currently the "Confidential" in confidential issues simply means that if a project is public, only those with Reporter access or above can see the issue. Many customers have asked for this to change so that Guest users who have been @mentioned in the issue can also see the issue.

There may be other methods of implementing this; but in general an issue type only visible to project owner (or perhaps some other level that could be configurable by issue) and subscribers would be welcome.