URN for image info (#78)

* Allow music service to return a URN identifying cover art for an entity

* Fix bug with playlist cover art rending same album multiple times

src/app.ts

@@ -88,7 +88,8 @@ const app = server(
     applyContextPath: true,
     logRequests: true,
     version,
-    tokenSigner: jwtSigner(config.secret)
+    tokenSigner: jwtSigner(config.secret),
+    externalImageResolver: artistImageFetcher
   }
 );