Delete coreAPI PageRoot component in favour of a default component in kolibri_app

[rtibbles]

Summary

Instead of the core API exposing a very thin PageRoot component that is just a wrapper around router-view, instead make an even thinner component that is just used by default.

References

Fixes #11729

Reviewer guidance

All affected pages should still render identically.

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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-0.16.0b10.dev0_git.118.ge4de1bbd.pex
Windows Installer (EXE)	kolibri-0.16.0b10.dev0+git.118.ge4de1bbd-unsigned.exe
Debian Package	kolibri_0.16.0b10.dev0+git.118.ge4de1bbd-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b10.dev0+git.118.ge4de1bbd-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b10.dev0+git.118.ge4de1bbd-0.1.0-debug.apk
TAR file	kolibri-0.16.0b10.dev0+git.118.ge4de1bbd.tar.gz
WHL file	kolibri-0.16.0b10.dev0+git.118.ge4de1bbd-py2.py3-none-any.whl

[cbum-dev]

Reviewed two changes.

[marcellamaki]

lgtm - manual QA confirms affected pages still render as previously

[marcellamaki]

Merging without Python unit tests on Mac OS (3.10), as this is a side effect of the recently updated check to run Python unit tests on Mac OS using 3.10, and the previously existing check (when this PR was opened, 3.6) passed.