Remove empty sections from quizzes on publish

[rtibbles]

Summary

• When saving a syncable exam, automatically remove any empty sections, in the save method.
• Move question_count setting to the save method of the AbstractExam model rather than in the serializer to ensure consistency (this was apparent when I edited question_sources in a test, but the question_count didn't update)
• Leave the data model version on the data passed to the frontend in the class summary API so that it can be used by the frontend
• Add a warning to users in the start quiz confirmation modal that empty sections will be deleted.

References

Fixes #12217

Reviewer guidance

1. Create a quiz, add some questions to one section, then create some blank sections.
2. Save the quiz.
3. Go back to edit it, and see that the empty sections are still there.
4. Start the quiz
5. See that the quiz only has one section after starting the quiz (this part might be trickier)

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Testing checklist

• 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
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• 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

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.16.2a0.dev0+git.1211.g065436b0-windows-setup-unsigned.exe
Debian Package	kolibri_0.16.2a0.dev0+git.1211.g065436b0-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.2a0.dev0+git.1211.g065436b0-0.4.2.dmg
Android Package (APK)	kolibri-0.16.2a0.dev0+git.1211.g065436b0-0.1.3-debug.apk
TAR file	kolibri-0.16.2a0.dev0+git.1211.g065436b0.tar.gz
WHL file	kolibri-0.16.2a0.dev0+git.1211.g065436b0-py2.py3-none-any.whl

[nucleogenesis]

Chatted through on screenshare, all questions answered and the code & tests LGTM