Use gitlab-ui dropdown components for issue board switcher

This is a follow-up of https://gitlab.com/gitlab-org/gitlab-ee/issues/7715. We should switch to the gitlab-ui dropdown components (gitlab-ui!83 (merged)) after they have been merged. This adds

• switch board header to the dropdown as in the design of https://gitlab.com/gitlab-org/gitlab-ee/issues/7715
• keyboard navigation
• fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/47721
• reduces complexity of the board switcher component