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.

spec/system/admin/widgets/cards_spec.rb

@@ -163,7 +163,7 @@ describe "Cards", :admin do
         fill_in "Title", with: "Card for a custom page"
         click_button "Create card"
 
-        expect(page).to have_current_path admin_site_customization_page_cards_path(custom_page)
+        expect(page).to have_current_path admin_site_customization_page_widget_cards_path(custom_page)
         expect(page).to have_content "Card for a custom page"
       end
 
@@ -199,7 +199,7 @@ describe "Cards", :admin do
       scenario "Edit", :js do
         create(:widget_card, page: custom_page, title: "Original title")
 
-        visit admin_site_customization_page_cards_path(custom_page)
+        visit admin_site_customization_page_widget_cards_path(custom_page)
 
         expect(page).to have_content("Original title")
 
@@ -211,7 +211,7 @@ describe "Cards", :admin do
 
         click_button "Save card"
 
-        expect(page).to have_current_path admin_site_customization_page_cards_path(custom_page)
+        expect(page).to have_current_path admin_site_customization_page_widget_cards_path(custom_page)
         expect(page).to have_content "Updated title"
         expect(page).not_to have_content "Original title"
       end
@@ -219,7 +219,7 @@ describe "Cards", :admin do
       scenario "Destroy", :js do
         create(:widget_card, page: custom_page, title: "Card title")
 
-        visit admin_site_customization_page_cards_path(custom_page)
+        visit admin_site_customization_page_widget_cards_path(custom_page)
 
         expect(page).to have_content("Card title")
 
@@ -227,7 +227,7 @@ describe "Cards", :admin do
           click_link "Delete"
         end
 
-        expect(page).to have_current_path admin_site_customization_page_cards_path(custom_page)
+        expect(page).to have_current_path admin_site_customization_page_widget_cards_path(custom_page)
         expect(page).not_to have_content "Card title"
       end
     end