Allow tutorials to be localized

[ewpatton]

To test this functionality, you will need to build App Inventor from the branch feature/tutorial-translations-debug, which has PR mit-cml/appinventor-sources#2188 plus support for allowing localhost URLs in the tutorial URL.

I used Google Translate to translate the markdown tutorial from English into Simplified Chinese (@ellelili2025 should probably review it because I expect it's pretty bad).

This works similar to how the documentation localization works. The aforementioned PR adds a locale parameter if the user is using a language other than English. The changes in util.js will detect this, test to see if a page exists with the language tag before the ".html", and will redirect if so.

To test:

1. Run bundle exec jekyll serve from the root
2. Start your test server using the feature/tutorial-translations-debug branch
3. Load HelloPurr in your test server from the no projects dialog
4. Paste this URL into the TutorialURL: http://localhost:4000/yrtoolkit/yr/tutorials/markdown-tutorial.html
5. Confirm the tutorial loads in the panel
6. Switch the language to Simplified Chinese (first Chinese option)
7. After App Inventor reloads, confirm that the tutorial panel now shows you the Chinese translation

Change-Id: I564c76c2fd058592a5ec7fb4eb551b63b0b04cc3