Refactor frontend-only fields and defensiveness

[bjester]

Summary

Description of the change(s) you made

• As shown in the screenshot below, the frontend was creating change events and syncing them for fields that only have meaning on the frontend
• This PR adds defensive checks at several levels to prevent syncing of fields that are only used on the frontend

Manual verification steps performed

1. Open the developer console for tracking syncs
2. Copy an exercise resource
3. Open the new copy for editing, but don't edit anything
4. Verify you don't see any frontend fields, like isNew or __last_fetch, being synced to the backend

Are there any risky areas that deserve extra testing?

• Exercise / assessment editing

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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]

All code changes make sense on read through, doing manual testing now.

[rtibbles]

Manually tested both new node creation and question creation, I did not see any superfluous/erroneous syncing events, and syncs of actual edits all seemed to go as they should.