Secured the user management rules.

Removed Role and Resources from resources listing if user is not at
System level;
Added guard in Role pre-save to exclude Role and Resources access
tokens if the user is not at System level;
Added guard in privilege map calculation in User model to make sure the
role objects in roles are from or below the space of this user's
creator but never above.
Changed space levels to Void->System->Administrator->User.