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

tests/music_service.test.ts

@@ -0,0 +1,22 @@
+import { v4 as uuid } from "uuid";
+
+import { anArtist } from "./builders";
+import { artistToArtistSummary } from "../src/music_service";
+
+describe("artistToArtistSummary", () => {
+  it("should map fields correctly", () => {
+    const artist = anArtist({
+      id: uuid(),
+      name: "The Artist",
+      image: {
+        system: "external",
+        resource: "http://example.com:1234/image.jpg",
+      },
+    });
+    expect(artistToArtistSummary(artist)).toEqual({
+      id: artist.id,
+      name: artist.name,
+      image: artist.image,
+    });
+  });
+});