make sure the question count v-model uses .number

[marcellamaki]

Summary

Fixes #12406 by adding .number to the questionCount v-model. I think this is okay, but alternatively I could update the increment to parseInt() if that's preferable

References

add-not-concat.mp4

Reviewer guidance

In develop, input a number of questions typing into the textbox (i.e. 5) and then click the "+" button. You should see 51, rather than 6.
Checkout PR, and do the same. It should add the value, not concat as strings

Testing checklist

• Contributor has fully tested the PR manually
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Reviewer checklist

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[rtibbles]

I think this is okay, but alternatively I could update the increment to parseInt() if that's preferable

I think this is exactly what .number is intended to do: https://v2.vuejs.org/v2/guide/forms#number

It also guarantees that we're emitting a consistent data type always (because if we left the type coersion just to when we press the buttons, then we're still going to have a string sometimes).

[rtibbles]

Code changes make sense, video and manual testing confirms the fix.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.17.0a0.dev0+git.223.g89b11c0e-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.223.g89b11c0e-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.223.g89b11c0e-0.4.2.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.223.g89b11c0e-0.1.3-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.223.g89b11c0e.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.223.g89b11c0e-py2.py3-none-any.whl