Right now, when there are visible groups we always select "All participants" at start, we should probably select the first group the student belongs to.

In MOBILE-4031 we decided that, when prefetching a forum, only the All participants group is prefetched if the user can view it. So when this issue is implemented we should either change the prefetch to also download current group in this case, or make forum fallback to all participants if the user group fails.