Dynamic redirects based on language

[acelaya]

Summary

Allow dynamically redirecting to different long URLs based on the visitor's language (potentially from Accept-Language header).

Depends on #1914

Use case

[acelaya]

There's some considerations around languages. For example, according to MDN, a typical Accept-Language header value could look like this Accept-Language: fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5. That means that:

• The * language should be ignored when it comes to match a redirect condition, as it would always match no matter what.
• If the Accept-Language header is empty, not present, or only has the value *, the condition will not be considered a match.
• Values can be defined as language codes (fr, en, it, etc) or full locales (fr-FR, en-US, es-AR). Both should be allowed as matchValue, and if only the language code is used for matching, any locale for that language should resolve a positive match (es will match es-ES, es-AR, es-MX, etc. while en-UK will not match en-US)
• The quality part should be ignored (q=0.5), as we want to check if the language/locale defined as matchValue is present in any of the languages.