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/auth/open_id_authenticator.rb

@@ -10,7 +10,7 @@ class Auth::OpenIdAuthenticator < Auth::Authenticator
   end
 
   def enabled?
-    SiteSetting.send(@enabled_site_setting)
+    SiteSetting.get(@enabled_site_setting)
   end
 
   def description_for_user(user)