Add filter to issue boards switcher dropdown

Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/issues/48847 and https://gitlab.com/gitlab-org/gitlab-ce/issues/40219. Originally proposed in https://gitlab.com/gitlab-org/gitlab-ce/issues/40298#note_48542666

One of the biggest challenges currently in issue boards is navigating between multiple boards. We can make this a bit easier by adding a search input to the boards switcher dropdown:

• Design specs
• Component docs: Search and Dropdowns
• The filter field should be focused when opening the dropdown.
• Display No matching boards found when filtering didn't return any results

This is a small effort for a big gain in UX. It does not solve the root problem of duplicated and unused boards, but it definitely alleviates that issue.

Implementation

• moving the Vue template out of HAML: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8519
• replace the dropdown implementation with gitlab-ui
  • add dropdown components to gitlab-ui: gitlab-ui!83 (merged)
  • use new components in the switcher: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8591
  • afterwards keyboard navigation works and Switch board header is added
• add search field to dropdown
  • add search field to gitlab-ui: gitlab-ui!96 (merged)
  • afterwards new merge request in EE for updating the dropdown

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