Add only users in group can start a topic as permissions option for forum

[jeffputz]

Forums can currently be set so that only users in a group can view, or only users in a group can post. Add a third option where only users in a group can start a topic.

[jeffputz]

The UI for this may be a little challenging. Right now, the roles can be "moved" left and right into the view and post columns in the admin.