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/discourse.rb

@@ -84,6 +84,8 @@ module Discourse
   # Cross site request forgery
   class CSRF < StandardError; end
 
+  class Deprecation < StandardError; end
+
   def self.filters
     @filters ||= [:latest, :unread, :new, :read, :posted, :bookmarks]
   end