Move Supported Languages to year config

[tunetheweb]

Currently Support Languages is defined in two places:

1. In validate.py for the Python scripts.
2. In generate_chapters.js for generating the sitemap.

This also means it’s set once at project level when we might have different languages supported in different years.

We should move this to the year-specific JSON config so we once config isn’t forgotten and so can support different languages per year.

[tunetheweb]

This has been moved to JSON config now as part of #712 :

{
  "settings": [
    {
      "isLive": true,
      "supported_languages": ["en","es","fr","ja"]
    }
  ],

However we still need to update the generate_chapter.js to use this instead of hardcoding this:

//Chapters may exist but not be ready to be launched so do not include in sitemap
const sitemap_languages = ['en','es','fr','ja'];

We also need to make it year specific so need to update generate_sitemap.js for this. Still think we should do the load in generate_chapter.js (as the config might be used for other things in future) but it should be set in a variable that is then used in generate_sitemap.js.