Count descendants not ancestors

[rtibbles]

Summary

• Updates lesson resource page to fetch number of descendants for the currently displayed topic (a value we precompute during annotation)
• Use this to display selection metadata
• Adds selection metadata to channels as well
• Ensures that the resource cache is properly populated even on page refresh
• Does flyby fix of Lesson Creation back button behaviour, which on multi-class setups resulted in not going back to the intended page

References

Fixes #12387

Reviewer guidance

Manage the resources in a lesson, ensure that when resources are selected "X of Y resources selected" is always properly displayed

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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-0.17.0a0.dev0+git.179.g8f2534fc-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.179.g8f2534fc-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.179.g8f2534fc-0.4.2.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.179.g8f2534fc-0.1.3-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.179.g8f2534fc.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.179.g8f2534fc-py2.py3-none-any.whl

[pcenov]

LGTM no issues observed while manually testing.