Handle partially imported channels in the device plugin

[rtibbles]

Summary

• Channels can now be partially imported which means that the device plugin might still need to import the full channel metadata even if some channel metadata is present
• Fixes this by ensuring that a partially imported channel will still be imported by a channel import task - and adds a test to be sure of this
• Also by default filters requests for a single channels metadata for partial=false to ensure that only fully imported channels are displayed
• Exempts the channel status display page from this, as it is just showing the currently imported channel state, so the partial import is sufficient - all other usages seem to be for importing more, so need the full metadata.

References

Fixes #11185

Note that this slightly exacerbates the situation for #11225 as it will wipe any previously annotated admin_imported=False from anywhere, but I am assigned to that issue too, so it's my problem to clean up there :)

Reviewer guidance

Ensure that partially imported channels (such as that produced by learner downloads or syncing of lessons/quizzes) do not prevent additional resources being imported in those channels.

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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

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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-0.16.0b5.dev0_git.104.g9d1f445f.pex
Windows Installer (EXE)	kolibri-0.16.0b5.dev0+git.104.g9d1f445f-unsigned.exe
Debian Package	kolibri_0.16.0b5.dev0+git.104.g9d1f445f-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b5.dev0+git.104.g9d1f445f-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b5.dev0+git.104.g9d1f445f-0.1.0-debug.apk
TAR file	kolibri-0.16.0b5.dev0+git.104.g9d1f445f.tar.gz
WHL file	kolibri-0.16.0b5.dev0+git.104.g9d1f445f-py2.py3-none-any.whl

[jredrejo]

LGTM