Enable content localization for iOS summarizer landing page

[janbrasna]

One-line summary

Reverts subsetting the content in just en-US — to enable l10n tooling for the landing page, and subsets the next expansion languages in configs. (Following up from #464 …)

Significant changes and points to review

Will also need deleting the en-US file in l10n repo(s) separately, as the automation doesn't do removals.

This changeset does not try to subset Smartling languages, as these always trigger all anyways. Only subsets Pontoon locales.

The expansion is currently planned to DE, FR, ES, PT (and possibly JP).

This PR will enable these languages:

1. Smartling: "ar", "de", "fr", "hi-IN", "id", "ja", "ms" (it's all or nothing only)
2. Pontoon: "en-CA", "en-GB", "es-AR", "es-CL", "es-ES", "es-MX", "pt-BR", "pt-PT"

Issue / Bugzilla link

mozilla-l10n/www-firefox-l10n#72 (comment)

Testing

http://localhost:8000/en-US/landing/ios-summarizer/ (control, no change)

http://localhost:8000/en-CA/landing/ios-summarizer/ (should not redirect to en-US)
http://localhost:8000/de/landing/ios-summarizer/ (should not redirect to en-US)
http://localhost:8000/es-AR/landing/ios-summarizer/ (should not redirect to en-US)
http://localhost:8000/ar/landing/ios-summarizer/ (should not redirect to en-US)
http://localhost:8000/hi-IN/landing/ios-summarizer/ (should not redirect to en-US)

http://localhost:8000/pl/landing/ios-summarizer/ (should redirect to default)
http://localhost:8000/sco/landing/ios-summarizer/ (should redirect to default)

[janbrasna]

FYI @issammani this is waiting for when you need to start localizing https://www.firefox.com/landing/ios-summarizer/ for other languages as well — if you plan to use the page further and wanted to open to all locales. Just ping to get this in motion. 🚀

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.52%. Comparing base (be8c8c3) to head (2016531).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #476      +/-   ##
==========================================
- Coverage   81.72%   79.52%   -2.20%     
==========================================
  Files         144      147       +3     
  Lines        9099     9403     +304     
==========================================
+ Hits         7436     7478      +42     
- Misses       1663     1925     +262     

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