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
Edited by Winnie Hellmann