Refactor labels selector in board scope

app/assets/javascripts/boards/boards_util.js

app/assets/javascripts/boards/components/board_form.vue

app/assets/javascripts/boards/components/boards_selector.vue

app/assets/javascripts/boards/index.js

...sets/javascripts/boards/mount_multiple_boards_switcher.js

...s/graphql_shared/queries/group_users_search.query.graphql

..._shared/queries/project_user_members_search.query.graphql

...d/components/dropdown/dropdown_widget/dropdown_widget.vue

...ar/labels_select_widget/dropdown_contents_create_view.vue

...mponents/sidebar/labels_select_widget/dropdown_header.vue

...r/labels_select_widget/graphql/group_labels.query.graphql

...labels_select_widget/graphql/project_labels.query.graphql

app/views/shared/boards/_switcher.html.haml

.../assets/javascripts/boards/components/assignee_select.vue

ee/app/assets/javascripts/boards/components/board_scope.vue

...pp/assets/javascripts/boards/components/labels_select.vue

...assets/javascripts/boards/components/milestone_select.vue

ee/spec/features/boards/scoped_issue_board_spec.rb

ee/spec/features/epic_boards/epic_boards_spec.rb

ee/spec/features/labels_hierarchy_spec.rb