This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
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
