[Forum] Only users with management rights can add comments after a user has participated to the topic

On a topic without moderation, after a user added a comment, only the managers can add comments.

To reproduce:
1. In a forum, User A (write rights on the workspace) creates a topic without moderation.
2. User A adds a comment on the topic.
3. User B (write rights on the workspace too) goes on the topic to reply to the comment.
-> He doesn't have the "Reply" link nor the "Add" button. So he can't participate to the topic.
4. User C (management rights on the workspace) goes ont the topic.
-> He has both "Reply" link and "Add" button.