Hide Difficult questions tab if the quiz is still a draft

[AlexVelezLl]

Summary

Hide Difficult questions tab if the exam is still a draft.

non-draft quiz:

draft quiz:

References

Closes #12431.

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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]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.17.0b0.dev0+git.11.gc16faea6-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0b0.dev0+git.11.gc16faea6-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0b0.dev0+git.11.gc16faea6.dmg
Android Package (APK)	kolibri-0.17.0b0.dev0+git.11.gc16faea6-0.1.3-debug.apk
TAR file	kolibri-0.17.0b0.dev0+git.11.gc16faea6.tar.gz
WHL file	kolibri-0.17.0b0.dev0+git.11.gc16faea6-py2.py3-none-any.whl

[marcellamaki]

Code changes look good to me, I'll wait for @pcenov's manual QA approval though. Thanks, @AlexVelezLl !!

[pcenov]

Thanks @AlexVelezLl - no issues observed while manually testing, the tab is hidden now.