Add Django CMS 4 support

[stefanw]

Description

This PR makes djangocms-transfer roughly work with Django CMS 4.1 installations.
We needed import/export between different installations and this works for us.

Maybe this can be a starting point for a 4.1 compatible release.

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[fsbraun]

@stefanw Thanks for this work. Looks good at first sight! I will review in more detail, but

• Uses API to create plugins 👍
• Respects CMS 4 toolbar object 👍 (here, we might need to look at separate code for v3 and v4)