Make widget cards polymorphic

So now we'll be able to add them to other sections.

We're also adding a `dependent: :destroy` relation to models having
cards since it doesn't make sense to have cards around when their page
has been destroyed.

config/routes/admin.rb

@@ -271,7 +271,7 @@ resolve "Audit" do |audit|
 end
 
 resolve "Widget::Card" do |card, options|
-  [*resource_hierarchy_for(card.page), card]
+  [*resource_hierarchy_for(card.cardable), card]
 end
 
 resolve "Budget::Group" do |group, options|