Align dropdown component with spec part 2

This MR updates the new-dropdown component to further align with the specs: https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/dropdown-spec-previews/. The new-dropdown component will eventually replace the current dropdown component.

Changes:

• The new-dropdown-check-item was removed
• The new-dropdown-item now has the ability to show:
  • A check mark
  • A gitlab-ui svg icon and color to the left
  • An avatar
  • Secondary text
  • A gitlab-ui svg icon to the right
• Updated examples and stories

Deployment to view the code changes: http://cngo-align-dropdown-component-with-spec-part-2.162.243.131.170.nip.io/?path=/story/base-new-dropdown--default

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

Screenshots

Stories before	Stories after

Added to new-dropdown: