refactor fallback locale 🌋

[darcywong00]

Addresses review comment #733 (comment) by taking a subset of those changes

I think we want our URLs to redirect if a fallback is encountered, rather than accepting es-Fooo-BA as a substitute for es. And we don't want case insensitivity either; we don't want es, eS, Es, and ES all to return the same document, because URLs are case sensitive. So ...

• Locale::setLocaleFromURL() changes
  • tweak the regex to be case-sensitive (capitalize script and region)
  • redirect to the fallback locale (if found)

This way, es-Fooo-BA and es-ES redirect to es.

Unspecified locale still redirects to en

Test-bot: skip

[keymanapp-test-bot]

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