Handle suspicious file operations and return a 404.

[rtibbles]

Summary

• Django already smartly handles attempts to traverse the file system with URL paths
• We now catch these errors and return a 404
• Adds a regression test for this behaviour

References

Fixes #11595

Reviewer guidance

Do the changes make sense? Does the test make sense?

---

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]

LGTM

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b9.dev0_git.48.g03e3c0f2.pex
Windows Installer (EXE)	kolibri-0.16.0b9.dev0+git.48.g03e3c0f2-unsigned.exe
Debian Package	kolibri_0.16.0b9.dev0+git.48.g03e3c0f2-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b9.dev0+git.48.g03e3c0f2-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b9.dev0+git.48.g03e3c0f2-0.1.0-debug.apk
TAR file	kolibri-0.16.0b9.dev0+git.48.g03e3c0f2.tar.gz
WHL file	kolibri-0.16.0b9.dev0+git.48.g03e3c0f2-py2.py3-none-any.whl