Add Backwards compatibility with solidus_affirm

[peterberkenbosch]

Ideally this extension is a drop-in replacement for solidus_affirm, removing solidus_affirm from existing stores is causing some issues with existing orders etc.

This issue probably needs to be split up into a couple of smaller issues and pull requests.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.