Add `annotation-user` helper module

Add helper module with function for determining what name string to
use when rendering the author of a given annotation (display name or
username)

src/sidebar/helpers/annotation-user.js

+/**
+ * @typedef {import("../../types/api").Annotation} Annotation
+ */
+
+import { username } from './account-id';
+
+/**
+ * What string should we use to represent the author (user) of a given
+ * annotation: a display name or a username?
+ *
+ * The nice, human-readable display name should be used when a display_name
+ * is available on the annotation AND:
+ * - The author (user) associated with the annotation is a third-party user, OR
+ * - The `client_display_names` feature flag is enabled
+ *
+ * Return the string that should be used for display on an annotation: either the
+ * username or the display name.
+ *
+ * @param {Pick<Annotation, 'user'|'user_info'>} annotation
+ * @param {boolean} isThirdPartyUser - Is the annotation's user third-party?
+ * @param {boolean} isFeatureFlagEnabled - Is the `client_display_names`
+ *   feature flag enabled
+ * @returns {string}
+ */
+export function annotationDisplayName(
+  annotation,
+  isThirdPartyUser,
+  isFeatureFlagEnabled
+) {
+  const useDisplayName = isFeatureFlagEnabled || isThirdPartyUser;
+  return useDisplayName && annotation.user_info?.display_name
+    ? annotation.user_info.display_name
+    : username(annotation.user);
+}

src/sidebar/helpers/test/annotation-user-test.js

+import { annotationDisplayName } from '../annotation-user';
+
+describe('sidebar/helpers/annotation-user', () => {
+  const fakeAnnotations = {
+    withDisplayName: {
+      user: 'acct:albert@victoriana.com',
+      user_info: { display_name: 'Albert, Prince Consort' },
+    },
+    noDisplayName: {
+      user: 'acct:albert@victoriana.com',
+      user_info: {},
+    },
+    noUserInfo: {
+      user: 'acct:albert@victoriana.com',
+    },
+  };
+
+  [
+    {
+      annotation: fakeAnnotations.withDisplayName,
+      isThirdParty: false,
+      isFeatureEnabled: false,
+      expected: 'albert',
+    },
+    {
+      annotation: fakeAnnotations.withDisplayName,
+      isThirdParty: true,
+      isFeatureEnabled: false,
+      expected: 'Albert, Prince Consort',
+    },
+    {
+      annotation: fakeAnnotations.withDisplayName,
+      isThirdParty: false,
+      isFeatureEnabled: true,
+      expected: 'Albert, Prince Consort',
+    },
+    {
+      annotation: fakeAnnotations.withDisplayName,
+      isThirdParty: true,
+      isFeatureEnabled: true,
+      expected: 'Albert, Prince Consort',
+    },
+    {
+      annotation: fakeAnnotations.noDisplayName,
+      isThirdParty: true,
+      isFeatureEnabled: true,
+      expected: 'albert',
+    },
+    {
+      annotation: fakeAnnotations.noUserInfo,
+      isThirdParty: true,
+      isFeatureEnabled: true,
+      expected: 'albert',
+    },
+  ].forEach(testCase => {
+    it('should return the appropriate author string for an annotation', () => {
+      assert.equal(
+        annotationDisplayName(
+          testCase.annotation,
+          testCase.isThirdParty,
+          testCase.isFeatureEnabled
+        ),
+        testCase.expected
+      );
+    });
+  });
+});