FEATURE: Site settings defaults per locale

This change-set allows setting different defaults for different locales. 

It also:

- Adds extensive testing around site setting validation

- raises deprecation error if site setting has the default property based on env

- relocated site settings for dev and tests in the initializer

- deprecated client_setting in the site setting's loading process

- ensure it raises when a enum site setting being set

- default_locale is promoted to `required` category.

- fixes incorrect default setting and validation

- fixes ensure type check for site settings

- creates a benchmark for site setting

- sets reasonable defaults for Chinese

lib/site_settings/yaml_loader.rb

@@ -1,32 +1,26 @@
 module SiteSettings; end
 
 class SiteSettings::YamlLoader
-
   def initialize(file)
     @file = file
   end
 
-  def env_val(value)
-    if value.is_a?(Hash)
-      value.has_key?(Rails.env) ? value[Rails.env] : value['default']
-    else
-      value
-    end
-  end
-
   def load
     yaml = YAML.load_file(@file)
     yaml.each_key do |category|
       yaml[category].each do |setting_name, hash|
         if hash.is_a?(Hash)
           # Get default value for the site setting:
-          value = env_val(hash.delete('default'))
-
-          if hash.key?('hidden')
-            hash['hidden'] = env_val(hash.delete('hidden'))
+          value = hash.delete('default')
+          if value.is_a?(Hash)
+            raise Discourse::Deprecation, "Site setting per env is no longer supported. Error setting: #{setting_name}"
           end
 
-          yield category, setting_name, value, hash.symbolize_keys!
+          if hash['hidden']&.is_a?(Hash)
+            raise Discourse::Deprecation, "Hidden site setting per env is no longer supported. Error setting: #{setting_name}"
+          end
+
+          yield category, setting_name, value, hash.deep_symbolize_keys!
         else
           # Simplest case. site_setting_name: 'default value'
           yield category, setting_name, hash, {}