Add class `card` along with `card-slim` for Related Items apps

What does this MR do?

Fixes a regression where elements using card-slim but not using card along render broken card styles. This affected following Vue apps;

• ee/app/assets/javascripts/related_issues
• ee/app/assets/javascripts/related_items_tree

Screenshots

Before	After

Related Issues/MRs

https://gitlab.com/gitlab-org/gitlab-ce/issues/59875

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30748

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12925