Port to Backdrop CMS

Thank you for creating this module.

I have ported the Drupal 7 branch of this project to Backdrop CMS. It can be found here:

• https://backdropcms.org/project/views_sort_expression
• https://github.com/backdrop-contrib/views_sort_expression

A corresponding issue to this issue can be found at:
https://github.com/backdrop-ops/contrib/issues/865