Refactor JSON templates to be generated with ActiveModelSerializers instead of Rabl (#4090)

app/serializers/rest/preview_card_serializer.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class REST::PreviewCardSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :url, :title, :description, :type,
+             :author_name, :author_url, :provider_name,
+             :provider_url, :html, :width, :height,
+             :image
+
+  def image
+    object.image? ? full_asset_url(object.image.url(:original)) : nil
+  end
+end