fix(l10n): ensure that supported languages are a subset of the default s...

...upported languages

This would prevent bugs like #1429 from going unnoticed. WIP, still need to make the configuration testable.
Ask @shane-tomlinson &mdash; looks like he added it in https://github.com/mozilla/fxa-content-server/commit/51e7a4e75d2ebe96477e7b4e50bb2371adf796c5