Add board lists to Group Export

Board lists are currently not exported in Group Import/Export.

When I was working on it - it was failing due to this validation http://gitlab.com/gitlab-org/gitlab/blob/georgekoltsov/group-structure-import/ee/app/models/ee/list.rb#L28-28 even though my license was valid and had this feature.

Investigate and add board lists to Group Export. Similar to Project Export https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/import_export/import_export.yml#L87-90