Where do dropdown buttons live in gitlab ui/pajamas

The following discussion from gitlab-org/gitlab-ui!1807 (merged) should be addressed:

• @cngo started a discussion: (+3 comments)

  @tauriedavis No problem, I've added a story to the base button as well: http://cngo-dropdown-loading-state.162.243.131.170.nip.io/?path=/story/base-button--dropdown-loading-button

  I've kept the loading state story in the dropdown too since as a developer I will also look at the dropdown section to look for dropdown loading states.

  Does everything look okay now?