Group selector unique css class

Background: Group selector is always shown to Admins/Moderators/TAs if course is cohorted, regardless of if **discussion** is cohorted. However, backend code just ignores `group_id` parameter if discussion is not cohorted. This PR adds a unique css class to group selector wrapper element, so this control can be targeted in css and hidden. It is a workaround to fix critical issue reported by a client before the release. Other part of fix is in named client private repository.

Partner information: 3rd party-hosted open edX instance, for an edX solutions client.

Merge deadline: before release is released.