Fix 500 errors when using django redis cache for response caching

[rtibbles]

Summary

• Fixes a small oversight whereby the passed in caching key generation function is not used consistently in the function decorator (not directly related to this fix, but important cleanup)
• Django Redis Cache does not implement an idiomatic cache backend for Django, inasmuch as it always returns a value for each of its cache methods.
• This causes particular issues both in our code and in the code used by the default Django cache middleware, whereby a response is cached in its post render callback, with the implementation using a lambda to invoke the cache setting, with the expectation that cache.set returns None.
• Due to the fact that the django-redis-cache implementation returns the response from the Redis client as the return of the set function (in the case of a successful set ' True) this is returned from the lambda and assumed by Django to be a replacement response object to be returned instead of the original. Hilarity ensues.
• This is fixed by subclassing the django-redis-cache RedisCache class and simply not returning anything from the set function while invoking the super method.

References

Fixes #11848

Reviewer guidance

Load an uncached contentnode API endpoint and confirm it does not return a 500.

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[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b13.dev0_git.13.gfd644f4b.pex
Windows Installer (EXE)	kolibri-0.16.0b13.dev0+git.13.gfd644f4b-unsigned.exe
Debian Package	kolibri_0.16.0b13.dev0+git.13.gfd644f4b-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b13.dev0+git.13.gfd644f4b-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b13.dev0+git.13.gfd644f4b-0.1.0-debug.apk
TAR file	kolibri-0.16.0b13.dev0+git.13.gfd644f4b.tar.gz
WHL file	kolibri-0.16.0b13.dev0+git.13.gfd644f4b-py2.py3-none-any.whl

[jredrejo]

Tested in a RPi, it fixed the issue and everything worked perfectly.
Code looks good too.

[jredrejo]

Brilliant!

[jredrejo]

surprised by this... until I re-read the PR description