Add embed_url to preview cards (#5775)

app/models/preview_card.rb

@@ -21,6 +21,7 @@
 #  height             :integer          default(0), not null
 #  created_at         :datetime         not null
 #  updated_at         :datetime         not null
+#  embed_url          :string           default(""), not null
 #
 
 class PreviewCard < ApplicationRecord