[OJS] Journal Manager as author can see reviewers in discussion

[martijnende]

Describe the bug
When a user with Journal Manager permissions submits a manuscript, they can do so with Author-level permissions. In principle, this limits its visibility to reviewers and other participants. However, when this user starts a discussion thread, they can see and include all the participants, including the reviewers, by name.

To Reproduce
Steps to reproduce the behavior:

1. Create or log-in as a Journal Manager
2. Submit a manuscript as an "Author"
3. Let an editor assign reviewers or other participants
4. Let the author start a discussion thread

What application are you using?
OJS 3.3.0.13

Additional information
This issue is possibly related to #7800