Redirect language URLs with non-translated slugs

[distantnative]

Description

Summary of changes

• If a secondary-language page gets accessed with a URL that contains untranslated slugs (for which a translation exists), the request is redirected to the fully translated URL (302 redirect)

Reasoning

To make those pages only available under the main (full- translated) URL and not several ones (with untranslated slug parts).

Additional context

In #3550 (comment) under "Complex example", @lukasbestle asks for an additional unit test where the translated slug of a page is the same as the untranslated slug of another page. This unit test is not included here as it would fail. However, this behavior is also different before this PR: if a translated slug is the same as the untranslated slug of a different page, our router will return the different page. Moreover, our page rules also prevent creating a translated slug that matches an existing untranslated slug. This is why I didn't include this aspect in this PR. To me this seems as a much larger change to be discussed, while this PR implements the solution for #3550 based on the existing rules.

Changelog

Fixes

• Redirect requests to default language URI in secondary languages to the custom secondary language slug #3550

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion