Add iteration selector to board scope

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

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

ee/app/assets/javascripts/boards/boards_util.js

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

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

...ripts/boards/components/board_scope_current_iteration.vue

...assets/javascripts/boards/components/iteration_select.vue

ee/app/assets/javascripts/boards/constants.js

...s/javascripts/boards/graphql/board_scope.fragment.graphql

...avascripts/issues/list/queries/iteration.fragment.graphql

ee/spec/features/boards/scoped_issue_board_spec.rb

ee/spec/frontend/boards/boards_util_spec.js

ee/spec/frontend/boards/components/board_form_spec.js

ee/spec/frontend/boards/iteration_select_spec.js

ee/spec/frontend/boards/mock_data.js

ee/spec/frontend/boards/stores/actions_spec.js

locale/gitlab.pot

spec/frontend/boards/components/board_form_spec.js