Add doesntHavePermission and isNotAbleTo methods

[gdebrauwer]

In my projects, I use the hasPermission a lot, but I almost always inverse the result as I want to check if a user doesn't have a permission:

class PostPolicy
{
    public function create(User $user)
    {
        if (! $user->hasPermission('some_permission')) {
            return false;
        }

       // more if-statements that check other conditions

       return true;
    }
}

This PR adds dedicated a doesntHavePermission method (and isNotAbleTo method), so those kind of checks are a bit more readable:

class PostPolicy
{
    public function create(User $user)
    {
        if ($user->doesntHavePermission('some_permission')) {
            return false;
        }

       // more if-statements that check other conditions

       return true;
    }
}