Ensure that Kolibri can start regardless of whether redis and redis cache is installed.

[rtibbles]

Summary

• Fixes issue reported here: [DO NOT MERGE UNTIL 0.16.1] Fix Screen Flickering in Libarary page #11853 (comment)
• This was caused by the Android App not being able to import Redis packages
• Makes the redis packages optional and handle when they cannot be imported

Reviewer guidance

Easiest way to test this is from source, install all requirements, then do:
pip uninstall redis
Ensure that Kolibri still starts.
pip install redis
pip uninstall django-redis-cache
Ensure that Kolibri still starts.

Finally reinstall both packages, and set the following settings in options.ini:

[Cache]
CACHE_BACKEND = redis
CACHE_REDIS_MAXMEMORY_POLICY = allkeys-lru
CACHE_REDIS_MAXMEMORY = 1660575744

Make sure you have Redis setup locally, and ensure that Kolibri still starts properly.

Finally, install the APK and ensure that it now starts properly.

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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

[jredrejo]

Code looked good, but a minor change is needed for it to work

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b13.dev0_git.29.gad2ce830.pex
Windows Installer (EXE)	kolibri-0.16.0b13.dev0+git.29.gad2ce830-unsigned.exe
Debian Package	kolibri_0.16.0b13.dev0+git.29.gad2ce830-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b13.dev0+git.29.gad2ce830-0.4.0.dmg
Android Package (APK)	kolibri-0.16.0b13.dev0+git.29.gad2ce830-0.1.0-debug.apk
TAR file	kolibri-0.16.0b13.dev0+git.29.gad2ce830.tar.gz
WHL file	kolibri-0.16.0b13.dev0+git.29.gad2ce830-py2.py3-none-any.whl

[radinamatic]

APK is a go, no errors on my Motorola GPlay51!!! 🚀 🎉

[radinamatic]

Good to go on physical Android device! 💯