Allow MetaField::Subject 'top' attribute to be a function

[drn05r]

This may be a use case where what subjects to display should be restricted depending on the user. One example is when you have old and new divisional sub-tree. You don't want regular users to be able to use the old sub-tree divisions but you want editors/admins to be able to add these items to a divisions.

Allowing MetaField::Subject top attribute to be a function is useful but it could have unintended side effects. A user that does not have access to some subjects for a field could unset these subjects if they are being used and not in the sub-tree they can see. Therefore, it is essential to set the workflow component to type Field::Subject

[drn05r]

Like for #389 this can be done in the workflow XML. It is probably best this is reverted so as not to leave two methods to do the same thing which may lead to confusion.