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.

app/controllers/concerns/admin/widget/cards_actions.rb

@@ -0,0 +1,59 @@
+module Admin::Widget::CardsActions
+  extend ActiveSupport::Concern
+  include Translatable
+  include ImageAttributes
+
+  def new
+    @card.header = header_card?
+    render template: "#{cards_view_path}/new"
+  end
+
+  def create
+    if @card.save
+      redirect_to_index
+    else
+      render template: "#{cards_view_path}/new"
+    end
+  end
+
+  def edit
+    render template: "#{cards_view_path}/edit"
+  end
+
+  def update
+    if @card.update(card_params)
+      redirect_to_index
+    else
+      render template: "#{cards_view_path}/edit"
+    end
+  end
+
+  def destroy
+    @card.destroy!
+    redirect_to_index
+  end
+
+  private
+
+    def card_params
+      params.require(:widget_card).permit(
+        :link_url, :button_text, :button_url, :alignment, :header, :columns,
+        translation_params(Widget::Card),
+        image_attributes: image_attributes
+      )
+    end
+
+    def header_card?
+      params[:header_card].present?
+    end
+
+    def redirect_to_index
+      notice = t("admin.site_customization.pages.cards.#{params[:action]}.notice")
+
+      redirect_to index_path, notice: notice
+    end
+
+    def cards_view_path
+      "admin/widget/cards"
+    end
+end