WIP: Clean up new dropdown styles

Open Winnie Hellmann requested to merge winh-cleanup-dropdowns into master

What does this MR do?

Cleans up the changes made by merge requests linked in https://gitlab.com/gitlab-org/gitlab-ce/issues/28921#note_36011920.

Why was this MR needed?

The new dropdown style was applied per page. This merge request activates it globally.


See linked merge requests in https://gitlab.com/gitlab-org/gitlab-ce/issues/28921#note_36011920.

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28921