Update backend change event generation to mark some changes as unpublishable

[rtibbles]

Summary

Description of the change(s) you made

• Adds an additional field to the backend Change model to flag some changes as unpublishable, defaulting to False
• Does this as two migration operations to not back fill the current table, but instead leave as null
• Excludes unpublishable changes from the unpublished changes query
• Automatically set changes to tables that do not constitute a publishable change as unpublishable - so only changes that directly affect the channel contents will be marked as publishable.
• Automatically set published changes as unpublishable
• Update all backend change generation that is only intended to signal for an update of frontend state, and not an actual edit of the channel contents, to save the changes as unpublishable
• Add new tests to make sure the publishable changes query operates as intended in a range of circumstances

Manual verification steps performed

Published a channel. Refreshed the page. Confirmed that it wasn't publishable if I made no further changes.

References

Fixes #4202

Comments

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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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• 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
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• Migrations are safe for a large db

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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

[bjester]

Code looks good, tests look good. It all makes sense