feat(cloud-function): prefer locale where page exist

[caugner]

Description

Refines the redirect logic when an MDN url without locale is accessed, preferably choosing a locale where the target page actually exists.

Motivation

Avoids redirects to 404s, when the page does not exist in the user's primary preferred locale, but in another accepted locale, or in English (as a fallback).

Additional details

Deployed to the test environment, and verified:

Before

$ xh -h GET https://developer.allizom.org/docs/Web/API/WebCodecs_API "Accept-Language: fr, en"
HTTP/2.0 302 Found
(…)
location: /fr/docs/Web/API/WebCodecs_API
(…)

$ xh -h GET https://developer.allizom.org/docs/Web/API/WebCodecs_API "Accept-Language: es, fr"
HTTP/2.0 302 Found
(…)
location: /es/docs/Web/API/WebCodecs_API
(…)

After

$ xh -h GET https://test.developer.allizom.org/docs/Web/API/WebCodecs_API "Accept-Language: fr, en"
HTTP/2.0 302 Found
(…)
location: /en-US/docs/Web/API/WebCodecs_API
(…)

$ xh -h GET https://test.developer.allizom.org/docs/Web/API/WebCodecs_API "Accept-Language: es, fr"
HTTP/2.0 302 Found
(…)
location: /en-US/docs/Web/API/WebCodecs_API
(…)

Related issues and pull requests

Fixes #79.