Improve Droplab dropdown CSS and accessibility.
- Consolidate CSS
- Improve HTML structure for accebility
- Add ARIA roles to buttons and menu items.
ref: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10018#note_28343932
Edited by Lukas Eipert
ref: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10018#note_28343932