feat(language-menu): add automatic redirect experiment

[caugner]

Summary

(MP-1242)

Fixes #275.
Fixes #2528.
Fixes #2724.
Fixes #6981.

Problem

1. Search engines may display MDN links to a locale that is not the user's preferred locale (even if the browser indicates this locale as a preferred language).
2. We show fallback links to en-US if a page is not available in a locale, but this means the user has to manually navigate back to their preferred locale.

Solution

We already have a preferredlocale cookie that we set when the user switches the locale in the user interface.

Use this cookie to automatically redirect the user to the preferred locale under these circumstances:

1. The user has set a preferred locale.
2. The requested page is NOT in the preferred locale.
3. The requested page is available in the preferred locale.

Caveat: Changing the locale directly in the address bar will still redirect back.

---

How did you test this change?

Ran npm start in /cloud-function, then tested locally with xh:

% xh -h localhost:5100/en-US/docs/Web
HTTP/1.1 200 OK

% xh -h localhost:5100/fr/docs/Web   
HTTP/1.1 200 OK

% xh -h localhost:5100/fr/docs/Web cookie:preferredlocale=en-US
HTTP/1.1 302 Found
Cache-Control: no-store
Location: /en-US/docs/Web

% xh -h localhost:5100/en-US/docs/Web cookie:preferredlocale=fr
HTTP/1.1 302 Found
Cache-Control: no-store
Location: /fr/docs/Web

[LeoMcA]

Code looks good, but I'm torn on the overall behaviour:

Obviously the current behaviour is an annoyance to many users, since we've received a number of reports about it, but a fix which results in pasting a link containing e.g. the fr locale in and being redirected to en-US because of prior behaviour feels wrong.

However, I don't see an elegant way to do this in some cases but not others, so this feels like the best approach for most people. Let's see if we get an influx of issues filed saying they want something like the old behaviour back.

I imagine most users who need to switch between locales a lot are in our close community (eng/content/l10n), and perhaps we should let them know ahead of time that setting the cookie to something like false will disable this behaviour - perhaps we should explicitly support that, and leave it untouched when using the dropdown menu in this case - that can be a follow up task/PR though.

[caugner]

pasting a link containing e.g. the fr locale in and being redirected to en-US because of prior behaviour feels wrong

I realize that this could indeed be an annoyance for us, when looking at issues, and for translators.

perhaps we should let them know ahead of time that setting the cookie to something like false will disable this behaviour - perhaps we should explicitly support that

I agree that we should support opt-out from this feature, maybe even via the UI from the locale dropdown.

[DonalMe]

fwiw I filled #11717
It might be duplicated in this ongoing effort but just in case