Provide complete safe current user entity to frontend

Currently, the gon object only contains current_user.id, current_user.username, current_user.name. For gitlab-foss#32464 (closed), gitlab-foss#22921 (closed) and gitlab-foss#30468 (closed) we need the current user's avatar. So we should instead serialize the complete UserEntity / API::Entities::UserBasic to gon.