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

lib/wizard/step_updater.rb

@@ -37,7 +37,7 @@ class Wizard
         value = Upload.get_from_url(value) || ''
       end
 
-      SiteSetting.set_and_log(id, value, @current_user) if SiteSetting.send(id) != value
+      SiteSetting.set_and_log(id, value, @current_user) if SiteSetting.get(id) != value
     end
 
     def apply_setting(id)