perms_via doesn't work well with scopes

[stefanmajoor]

If you use the perms_via on a view, both the original view as well as the view which delegates the views need to implement all the scopes. Meaning that both the original view needs all scopes from the subviews, and the subviews need all the scopes from the original view.

Proposed solution:

If you set perms_via you are not allowed to implement scopes not present in the perms_via model view; only allow perms_via if the model is a superclass of the perms_via model, and automatically copy the scope functions.

[stefanmajoor]

@daanvdk @sjamaan