Use mixin for new dropdown style

What does this MR do?

Replace individual chunks for the new dropdown style by a mixin.

This is a followup of

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13106
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13183
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13184

This also fixes the top margin for the affected dropdowns as in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13183.

Why was this MR needed?

Too much duplicate code.

What are the relevant issue numbers?

#28921 (moved), #35424 (closed)

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13183#note_36442062