Fix a PHP notice on contribution thankyou page and add fixme comments…
… as settings code should be migrated
As comm_pref_in_thankyou
is not defined in getSettingsDefaults()
it throws a PHP notice on the contribution thankyou page when it has not been set.
Added some comments because the settings code really needs updating in this extension to use a settings/gdpr.setting.php file + newer get/set methods via \Civi::settings