Show display names for third-party users

Whenever possible, always show display names instead of usernames on
annotation cards if the creator of the card is a third-party user.

Even if the client_display_names feature flag is turned off, display
names _will_ still be shown for third-party users.

Display names will _not_ be shown for first-party users, unless the
client_display_names feature flag is on.

If the api_render_user_info feature flag is turned off in h, or if the
user doesn't have a display name, then of course the username must be
shown as the client has no access to a display name.

src/sidebar/components/annotation-header.js

@@ -14,7 +14,8 @@ function AnnotationHeaderController(features, groups, settings, serviceUrl) {
 
   this.displayName = () => {
     var userInfo = this.annotation.user_info;
-    if (features.flagEnabled('client_display_names')) {
+    var isThirdPartyUser = persona.isThirdPartyUser(this.annotation.user, settings.authDomain);
+    if (features.flagEnabled('client_display_names') || isThirdPartyUser) {
       // userInfo is undefined if the api_render_user_info feature flag is off.
       if (userInfo) {
         // display_name is null if the user doesn't have a display name.

src/sidebar/components/test/annotation-header-test.js

@@ -141,6 +141,42 @@ describe('sidebar.components.annotation-header', function () {
           isThirdPartyUser: false,
           expectedResult: 'Bill Jones',
         },
+        {
+          context: 'when the client_display_names feature flag is off but ' +
+                   'the user is a third-party user',
+          it: 'returns display_name even though client_display_names is off',
+          user_info: { display_name: 'Bill Jones' },
+          client_display_names: false,
+          isThirdPartyUser: true,
+          expectedResult: 'Bill Jones',
+        },
+        {
+          context: 'when client_display_names is on and the user is a ' +
+                   'third-party user',
+          it: 'returns the display_name',
+          user_info: { display_name: 'Bill Jones' },
+          client_display_names: true,
+          isThirdPartyUser: true,
+          expectedResult: 'Bill Jones',
+        },
+        {
+          context: 'when the user is a third-party user but the ' +
+                   'api_render_user_info feature flag is turned off in h',
+          it: 'returns the username',
+          user_info: undefined,
+          client_display_names: true,
+          isThirdPartyUser: true,
+          expectedResult: 'TEST_USERNAME',
+        },
+        {
+          context: "when the user is a third-party user but doesn't have a " +
+                   'display_name',
+          it: 'returns the username',
+          user_info: { display_name: null},
+          client_display_names: true,
+          isThirdPartyUser: true,
+          expectedResult: 'TEST_USERNAME',
+        },
       ].forEach((test) => {
         context(test.context, () => {
           it(test.it, () => {