Add scroll fade gradient to dropdown component
From https://design.gitlab.com/components/dropdowns#behavior:
If there are more rows than fit the dropdown, an overlay with a fade effect will be added at the top or bottom of the dropdown to indicate there are more rows to be scrolled. When reaching the end of the scrollable area, the overlay will be removed.
From https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/dropdown-spec-previews/: