Use polymorphic routes to manage cards

We use a different logic to load the card depending on the controller
we're using, and then share the rest of the code. This way we simplify
the code a bit, since we don't have to check for the page_id parameter.

config/routes/admin.rb

@@ -227,7 +227,7 @@ namespace :admin do
 
   namespace :site_customization do
     resources :pages, except: [:show] do
-      resources :cards, only: [:index]
+      resources :cards, except: [:show], as: :widget_cards
     end
     resources :images, only: [:index, :update, :destroy]
     resources :content_blocks, except: [:show]
@@ -270,6 +270,10 @@ resolve "Audit" do |audit|
   [*resource_hierarchy_for(audit.associated || audit.auditable), audit]
 end
 
+resolve "Widget::Card" do |card, options|
+  [*resource_hierarchy_for(card.page), card]
+end
+
 resolve "Budget::Group" do |group, options|
   [group.budget, :group, options.merge(id: group)]
 end