Port to Backdrop CMS

Hi @tanc

Thank you for your work on this module.

I have ported it to Backdrop CMS. It can be found at:
- https://github.com/backdrop-contrib/colectomy
- https://backdropcms.org/project/colectomy

A corresponding issue can be found at https://github.com/backdrop-ops/contrib/issues/739

If you would like to co-maintain, just let me know and I can add you the repo.

Kind regards
Martin