Fix URL prefixing for static urls

[rtibbles]

Summary

Gets rid of hardcoded URL for finding mathjax and uses static URL function.

References

Fixes #8358

Reviewer guidance

Run Kolibri with this env var set: KOLIBRI_URL_PATH_PREFIX="/kolibri"

Without this PR, it will 404 trying to load the Mathjax static file when rendering an exercise. With this PR it loads correctly.

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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
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• 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

[pcenov]

After installing the Windows build and running Kolibri with an env var set as KOLIBRI_URL_PATH_PREFIX="/kolibri" I don't see any issues in rendering the Spanish KA exercises. Let me know if there is a specific exercise which was failing so that I can confirm for it specifically.

[radinamatic]

Was this a Windows only issue? No 404s on MathJax files on Ubuntu with or without the env var set... 😀

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[rtibbles]

I was definitely able to replicate on Ubuntu without this fix, and then see it resolved with it.