Helper to create content data for kolibri tests

[jredrejo]

Summary

• Refactors some kolibri code to extract ChannelBuilder to an standalone file

References

Relates: https://github.com/learningequality/kolibri-data-portal/issues/630

Reviewer guidance

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-0.19.0a0.dev0_git.77.gd5717bb6.pex
Windows Installer (EXE)	kolibri-0.19.0a0.dev0+git.77.gd5717bb6-windows-setup-unsigned.exe
Debian Package	kolibri_0.19.0a0.dev0+git.77.gd5717bb6-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.19.0a0.dev0+git.77.gd5717bb6.dmg
Android Package (APK)	kolibri-0.19.0a0.dev0+git.77.gd5717bb6-0.1.4-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.19.0a0.dev0+git.77.gd5717bb6.zip
TAR file	kolibri-0.19.0a0.dev0+git.77.gd5717bb6.tar.gz
WHL file	kolibri-0.19.0a0.dev0+git.77.gd5717bb6-py2.py3-none-any.whl

[rtibbles]

This is a useful refactor by itself, and it's for test only code and tests still pass.