Fixes content_id bug whenever underlying content is changed 🎉

[vkWeb]

Summary

This PR fixes a long standing bug on studio wherein copied contentnodes content_id were not changed upon modification of their underlying content.

Particulary, exercises and file contentnodes were affected by this. Both the cases have been resolved by this PR.

Manual verification steps performed

1. Create a channel and a exercise contentnode with multiple assessment items.
2. Copy that exercise node.
3. Make sure the content_id of copied node is same as of the original node.
4. Now, try adding, editing or deleting any of the assessment items in the copied node, the content_id should be changed!
5. Repeat the following with the file contentnode type as well. That is, try modifying the uploaded file of a copied file node, you should see content_id updated.

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Reviewer guidance

References

Closes #1055
Closes #3112
Unblocks #3725

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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
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• If any Python requirements have changed, the updated requirements.txt files also included in this PR
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Studio-specifc:

• All user-facing strings are translated properly
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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]

I've laid out a case I think this fails to properly capture - some tests to cover these cases will help to document and validate the assumptions being made here.

[vkWeb]

@rtibbles I updated the content_id behavior according to the talk we had on slack.

For everyone else here is the summary of our consensus on how content_id should behave:
content_id of original node should not change on modifications to it. Copied node's content_id should change upon modification to its underlying content that is assessment items in case of exercises and files in case of file contentnodes.

@rtibbles @bjester The test cases should cover 100% of the changes I made and they test all possible scenarios I had in mind. I am very confident that this should be ready to merge! 🎉

[rtibbles]

This all makes sense to me, and my first read of the tests suggests to me that they are covering everything they should. I'd like to take another look to be sure, or a second pair of eyes would work too!

All my comments have been addressed.

[bjester]

Awesome tests! 💯