FEATURE: Localization admin settings config page (#31085)

This commit adds a new Localization config page for
admins, as a basic filtered site setting page similar
to Legal and Notifications. Included settings are:

* default locale
* allow user locale
* set locale from accept langauge header
* onebox locale
* display local time in user card
* discourse local dates enabled
* support mixed text direction
* unicode usernames
* allowed unicode username characters

app/models/site_setting.rb

@@ -9,6 +9,7 @@ class SiteSetting < ActiveRecord::Base
     fonts
     group_permissions
     legal
+    localization
     navigation
     notifications
     permalinks