DEV: improve design of site setting default provider

This refactors it so "Defaults provider" is only responsible for "defaults"

Locale handling and management of locale settings is moved back into
SiteSettingExtension

This eliminates complex state management using DistributedCache and makes
it way easier to test SiteSettingExtension

spec/components/site_setting_extension_spec.rb

@@ -505,6 +505,15 @@ describe SiteSettingExtension do
   end
 
   describe "shadowed_by_global" do
+
+    context "default_locale" do
+      it "supports adding a default locale via a global" do
+        global_setting :default_locale, 'zh_CN'
+        settings.default_locale = 'en'
+        expect(settings.default_locale).to eq('zh_CN')
+      end
+    end
+
     context "without global setting" do
       before do
         settings.setting(:trout_api_key, 'evil', shadowed_by_global: true)