Old group issue board URL doesn't redirect to new one

As of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3078, we’re redirecting /groups/:group_id/boards to /groups/%{group_id}/-/boards, but not /groups/:group/boards/:id, which means a bookmarks like https://gitlab.com/groups/gitlab-org/boards/378276 are now broken.

I think we should redirect /groups/:group_id/boards/*whatever to /groups/%{group_id}/-/boards/%{whatever}. We should also make sure that query parameters are correctly passed along, for bookmarks like https://gitlab.com/groups/gitlab-org/-/boards/378276?milestone_title=10.2&scope=all&utf8=%E2%9C%93&state=opened&label_name[]=Platform&label_name[]=Deliverable&label_name[]=backend that add some extra issue filters.

I don't know if we should treat this like a bug, or just a "nice to have", though.

/cc @smcgivern @victorwu @vsizov