text: Title: Implement urls domain, protocol and site name for frontend 
  Body: If you're develop a backend and frontend in separate projects, cannot add `ACTIVATION_URL` in DJOSER settings like this
  ```python
  DJOSER = {
      "SEND_ACTIVATION_EMAIL": True,
      "SEND_CONFIRMATION_EMAIL": True,
      "USER_CREATE_PASSWORD_RETYPE": True,
      # specially this
      "ACTIVATION_URL": os.environ.get(
          "DJANGO_ACTIVATION_URL", "auth/users/activate/{uid}/{token}"
      ),
  }
  ```
  because implicitly check inside the backend this path from example `localhost:8000`, and maybe the frontend project is in another repo and another host
  
  From documentation:
  ![image](https://github.com/sunscrapers/djoser/assets/78490028/dbe86144-c657-42af-b78a-0a06785fb1b0)
  
  
  This PR #729 by @n-borges allows this functionality
  Please check this out ❤️ 
  