standardizing page managers

I wanted to add a fade transition in between pages, but I realized that it gave the impression that performance was slightly slower. So there is no transition added here; I just extracted the common functionality into the page_manager_mixin and brought the landing page controller in line with the other two.
this is a sign that we'll need refactoring - the page manager is exploring various things inside inself (others?) to make an action. while this logic should be encapsulated in a simple call 

if (this.display()) {
  // do something else
}