Check against CanPlayerJoinClass

[Stanstar22]

So we can deny access to a class via a hook other than having to do it within class files per each class

Will only deny access to class if we return a value of false.

Extremely useful for checking against a mass of classes instead of having to set a check in each class file

[TovarischPootis]

This allows this hook hook.Run("CanPlayerJoinClass", client, class, info) to be run externally apart from the class files themselves. Also, the hook just needs to return false to do its function, which is far simpler and convenient