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/controllers/static_controller.rb

@@ -27,7 +27,7 @@ class StaticController < ApplicationController
 
     if map.has_key?(@page)
       site_setting_key = map[@page][:redirect]
-      url = SiteSetting.send(site_setting_key)
+      url = SiteSetting.get(site_setting_key)
       return redirect_to(url) unless url.blank?
     end