Replace AuthComponent usage with Authentication plugin.

This replaces the use of the deprecated AuthComponent with the Authentication plugin.

I chose to not use the Authorization plugin as app already relied on methods from GeneralFunctions and the GranularAccess components and rewriting them as authorization plugin policy would be a lot of unnecessary code churn with no practical benefit.

The current AppController::isAuthorized() has a bug which essentially renders it useless. It checks if prefix is admin but prefixes are now camel cased so it's Admin. That's why $this->GeneralFunction->checkIfRolesExists() were required in methods of every admin action. So I created a new RequestAccess component instead which properly handles access to admin actions and cleaned up all controllers.