Exclude non en-US users from download funnel experiment

[bobsilverberg]

As discussed at https://github.com/mozilla/bedrock/issues/10207#issuecomment-861355640, we need to limit the download funnel experiment to only en-US users, as anyone with a different language should not be directed to the new experimental download page.

We might be able to hack this into the current experiment code, but it might be better to add a feature to the framework that allows us to exclude users from an experiment based on language. This is something that has been discussed in https://github.com/mozilla/addons/issues/14193 and that we will want to have in place at some point, so now seems as good a time as any to do it.

I will open a separate issue specifically about allowing for configuring an experiment based on language.

[ioanarusiczki]

@bobsilverberg I verified this on AMO dev - with several languages:

• I used Chrome and cleared the funnel cookie from browser's settings, when the homepage is loaded in languages other than en-US, I see notInExperiment cookie set. Also this works with Edge - every time I opened a new Sandbox session and loaded the homepage in various locales. The locale of the installed browser are en-US.
• I tried on Sandbox with Chrome downloaded for my region (ro) and loaded AMO in en-US. In this situation I could see the funnel experiment cookie set to current or new-link for en-US. When I tried AMO with https://addons-dev.allizom.org/ro/firefox/, the user gets a notInExperiment cookie.

Important is what's the locale used into the URL when AMO is loaded the first time in the browsers:

• when https://addons-dev.allizom.org/en-US/firefox/ is loaded the funnel cookie is a current-link or new-link
• any other language except en-US gets a notInExperiment (this includes https://addons-dev.allizom.org/en-CA/firefox/ or https://addons-dev.allizom.org/en-GB/firefox/)

Verified on AMO dev - Chrome, Edge.