User-scoped channel quiz feature

[bjester]

Summary

Adds the ability to mark exercises as "channel quizzes," but only for admin users or users who have the feature enabled

Description of the change(s) you made

• Adds a feature flag for "channel quiz" functionality
• Adds a checkbox to toggle "channel quiz" modality only when editing an exercise
• Updates logic for saving content node extra fields so the new modality gets saved

Manual verification steps performed

1. Sign in as non-admin user
2. Open a channel that has exercises
3. Edit an exercise
4. Verify you do not see the new checkbox
5. In another browser or incognito session, sign in as an admin
6. As the admin, update the non-admin's account enabling the channel quiz feature
7. Refresh the non-admin's page
8. Edit an exercise
9. Observe the new checkbox
10. Enable the channel quiz functionality
11. Save your edits
12. Verify the change is saved in the database

Screenshots (if applicable)

Before / User-feature disabled	After / User-feature enabled

References

Resolves #3119

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Contributor's Checklist

Studio-specifc:

• All user-facing strings are translated properly

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 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 make sense, all manual verification steps worked.