Fix the CLI --pythonpath parameter

[rtibbles]

Summary

• Make setting django settings and pythonpath idempotent and reusable.
• Ensure we validate pythonpath option before settings option.
• Set python path for all CLI commands so that it can be used for plugins too.
• Cleans up deprecated methods

References

Fixes #10296

Reviewer guidance

Create a dummy plugin or settings file in a completely different folder. Pass the (can be relative) path to that folder using the the --pythonpath CLI option when invoking kolibri shell kolibri plugin enable or the like, and ensure that the settings file or plugin can take effect.

[github-actions[bot]]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0.dev0_git.20241121232341.pex
Windows Installer (EXE)	kolibri-0.18.0.dev0+git.20241121232341-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0.dev0+git.20241121232341-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0.dev0+git.20241121232341.dmg
Android Package (APK)	kolibri-0.18.0.dev0+git.20241121232341-0.1.4-debug.apk
TAR file	kolibri-0.18.0.dev0+git.20241121232341.tar.gz
WHL file	kolibri-0.18.0.dev0+git.20241121232341-py2.py3-none-any.whl