Backdrop port

Problem/Motivation

Create a version suitable for Backdrop.

Steps to reproduce

Currently listed as not available yet in Backdrop upgrade status, and cannot find anything while searching "reference" in the Backdrop modules.

Proposed resolution

Create the Backdrop module.

Remaining tasks

Run Coder Update on module to do initial changes.
Review changes.
Create new Readme.md.

User interface changes

None anticipated.

API changes

None anticipated.

Data model changes

None anticipated.