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

spec/requests/session_controller_spec.rb

@@ -267,11 +267,7 @@ RSpec.describe SessionController do
       SiteSetting.enable_sso = true
       SiteSetting.sso_secret = @sso_secret
 
-      # We have 2 options, either fabricate an admin or don't
-      # send welcome messages
       Fabricate(:admin)
-      # skip for now
-      # SiteSetting.send_welcome_message = false
     end
 
     let(:headers) { { host: Discourse.current_hostname } }