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.

spec/models/onebox_locale_site_setting_spec.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+RSpec.describe OneboxLocaleSiteSetting do
+  describe ".valid_value?" do
+    it "returns true for a locale that we have translations for" do
+      expect(OneboxLocaleSiteSetting.valid_value?("en")).to eq(true)
+    end
+
+    it "returns true for an empty value" do
+      expect(OneboxLocaleSiteSetting.valid_value?("")).to eq(true)
+    end
+
+    it "returns false for a locale that we do not have translations for" do
+      expect(OneboxLocaleSiteSetting.valid_value?("swedish-chef")).to eq(false)
+    end
+  end
+end