Align dropdown component with spec

There are two dropdown components: dropdown and new-dropdown — this MR updates the new-dropdown component which will replace the dropdown component.

This MR does the following things:

• Aligns the dropdown component with the spec here: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/dropdown-spec-previews/
• Renames various files and variables in the new-dropdown module to use a new-dropdown namespace to avoid leaking styles etc. with the dropdown component

Issue: gitlab-org/gitlab-services/design.gitlab.com#274 (closed)