DEV: introduce new API to look up dynamic site setting

This removes all uses of both `send` and `public_send` from consumers of
SiteSetting and instead introduces a `get` helper for dynamic lookup

This leads to much cleaner and safer code long term as we are always explicit
to test that a site setting is really there before sending an arbitrary
string to the class

It also removes a couple of risky stubs from the auth provider test

app/models/user.rb

@@ -1351,7 +1351,7 @@ class User < ActiveRecord::Base
     values = []
 
     %w{watching watching_first_post tracking muted}.each do |s|
-      category_ids = SiteSetting.send("default_categories_#{s}").split("|")
+      category_ids = SiteSetting.get("default_categories_#{s}").split("|")
       category_ids.each do |category_id|
         values << "(#{self.id}, #{category_id}, #{CategoryUser.notification_levels[s.to_sym]})"
       end