Resolve issues with public API migration

[bjester]

Summary

Description of the change(s) you made

• Removes tags that are longer than 30 chars from being exported to Kolibri public
• Only maps files for soruce nodes that were actually mapped and created
• Prevents adding children to non-topic nodes through ricecooker api

Manual verification steps performed

1. Download https://studio.learningequality.org/content/databases/b431ba9f16a3588b89700f3eb8281af0.sqlite3
2. Place downloaded sqlite file in .docker/minio/content/databases/
3. Run python contentcuration/manage.py export_channels_to_kolibri_public --channel-id b431ba9f16a3588b89700f3eb8281af0
4. Verify it completes successfully
5. Verify tests pass

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Reviewer guidance

How can a reviewer test these changes?

Manual steps above

References

Partially addresses: #4155
Resolves #4136
Resolves #4135

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Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
• If there new ways this uses user data that needs to be factored into our Privacy Policy, it has been noted.

Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

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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

[rtibbles]

Code changes look good, tests pass, and manual testing checks out.