FEATURE: Add a onebox_locale site setting. (#30655)

Following on from f369db5ae9a29a23299dff5f14768167230b0b79, this change adds the ability to choose a custom locale to send to onebox providers.

If this setting is left blank, it will fall back to using default_locale.

lib/onebox/helpers.rb

@@ -41,10 +41,11 @@ module Onebox
         canonical_uri = Addressable::URI.parse(canonical_link)
         if canonical_link && canonical_uri &&
              "#{canonical_uri.host}#{canonical_uri.path}" != "#{uri.host}#{uri.path}"
-          canonical_options = Oneboxer.get_final_destination_options(canonical_link)
-          canonical_options["extra_headers"] = { "Accept-Language" => accept_language }
-
-          uri = FinalDestination.new(canonical_link, canonical_options).resolve
+          uri =
+            FinalDestination.new(
+              canonical_link,
+              Oneboxer.get_final_destination_options(canonical_link),
+            ).resolve
           if uri.present?
             response =
               (
@@ -104,7 +105,7 @@ module Onebox
         headers ||= {}
 
         headers["User-Agent"] ||= user_agent if user_agent
-        headers["Accept-Language"] ||= accept_language if accept_language
+        headers["Accept-Language"] ||= Oneboxer.accept_language
 
         request = Net::HTTP::Get.new(uri.request_uri, headers)
         start_time = Time.now
@@ -236,14 +237,6 @@ module Onebox
       user_agent
     end
 
-    def self.accept_language
-      if SiteSetting.default_locale == "en"
-        "en;q=0.9, *;q=0.5"
-      else
-        "#{SiteSetting.default_locale.gsub(/_/, "-")};q=0.9, en;q=0.8, *;q=0.5"
-      end
-    end
-
     # Percent-encodes a URI string per RFC3986 - https://tools.ietf.org/html/rfc3986
     def self.uri_encode(url)
       return "" unless url