Locale files get precompiled after deployment and they contained translations from the `default_locale`. That's especially bad in multisites, because the initial `default_locale` is `en_US`. Sites where the `default_locale` isn't `en_US` could see missing translations. The same thing could happen when users are allowed to chose a different locale.
This change simplifies the logic by not using the `default_locale` in the locale chain. It always falls back to `en` in case of missing translations.
* English shouldn't fallback to any other locale
* Calculate fallback for default locale if it isn't English (useful for en_US)
* Reuse the fallback locale list when outputting translations to JavaScript
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
