Use templates for notification emails instead of PHP code

[Fenrikur]

📖 Story

As contributor to the Dealers' Den registration system, I want to be able to view, understand and edit the text for the email notifications without having to work with actual PHP code. Instead, the notification texts should be provided in a plain template format with placeholders.

✅ Acceptance Criteria

• Email notifications use templates which are stored in files with little or no PHP code.
  • PHP code would only be acceptable where variable values should be inserted into the template.

🙋 Open Questions

• How can this best be achieved with the tools available as part of the frameworks already in use?