Don't let users that are used for os user have their passwords set via the not specified password flow.

[rtibbles]

Summary

• Adds checks in the session creation endpoint and nonspecifiedpassword setting endpoint to exclude users for which there is an OSUser model
• Adds tests for the above

References

Fixes #12112

Reviewer guidance

This is probably best tested in the context of the Flatpak app @dylanmccall might be best placed to test this.

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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

[github-actions[bot]]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.2b1.dev0_git.2.g8bfe743e.pex
Windows Installer (EXE)	kolibri-0.16.2b1.dev0+git.2.g8bfe743e-windows-setup-unsigned.exe
Debian Package	kolibri_0.16.2b1.dev0+git.2.g8bfe743e-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.2b1.dev0+git.2.g8bfe743e-0.4.1.dmg
Android Package (APK)	kolibri-0.16.2b1.dev0+git.2.g8bfe743e-0.1.2-debug.apk
TAR file	kolibri-0.16.2b1.dev0+git.2.g8bfe743e.tar.gz
WHL file	kolibri-0.16.2b1.dev0+git.2.g8bfe743e-py2.py3-none-any.whl

[dylanmccall]

In learningequality/kolibri-installer-gnome#99, I changed the flatpak manifest to include this patch, and it appears to solve the problem I reported. There's one other issue over there which is probably because it isn't going through the initialize API. Reading the code, I see the app needs to use that for FacilityUser.objects.get_or_create_os_user() to run given the current auth token :) So I'll need to fix that and keep poking at it just to be sure, but this is looking good to me.