Add ancestor annotation to kolibri_public export mapper.

[rtibbles]

Summary

Description of the change(s) you made

• Adds ancestor annotation during the mapping process
• Adds a test for ancestor annotation during the mapping process

Manual verification steps performed

1. Reran the export to kolibri_public management command with Hsoub Academy
2. Verified that the ancestors were being properly returned from the API endpoint, e.g.:
   

References

Fixes #4177

Comments

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Contributor's Checklist

PR process:

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md