GraphQL: Destroy board mutation

Allow a user to destroy a board via GraphQL.

REST query:

DELETE /projects/:id/boards/:board_id

Code in ee/lib/ee/api/boards_responses.rb:

forbidden! unless board_parent.multiple_issue_boards_available?

            destroy_conditionally!(board) do |board|
              service = ::Boards::DestroyService.new(board_parent, current_user)
              service.execute(board)
            end