Add missing $type property to post subject

extension/background.js

···
     record: {
       $type: RECORD_TYPE,
       title: trimmedTitle,
-      subject: { url: trimmedUrl },
+      subject: { $type: "fyi.frontpage.feed.post#urlSubject", url: trimmedUrl },
       createdAt: new Date().toISOString()
     }
   };