group-pms: Update list when new group pm is made.

The function activity.process_loaded_messages(messages) would be called
from message_events.js, this would call people.huddle_string with the
same message object, it was expected that this would return a list of
ids but the message.display_recipient attribute which was being sent
here used a "user_id" field instead of an "id" field.

Fixes: #12503.

static/js/echo.js

@@ -83,11 +83,24 @@ function insert_local_message(message_request, local_id) {
             var person = people.get_by_email(email);
             if (person === undefined) {
                 // For unknown users, we return a skeleton object.
-                return {email: email, full_name: email,
+                return {
-                        unknown_local_echo_user: true};
+                    email: email,
+                    full_name: email,
+                    unknown_local_echo_user: true,
+                };
             }
+
             // NORMAL PATH
-            return person;
+            //
+            // This should match the format of display_recipient
+            // objects generated by the backend code in models.py,
+            // which is why we create a new object with a `.id` field
+            // rather than a `.user_id` field.
+            return {
+                id: person.user_id,
+                email: person.email,
+                full_name: person.full_name,
+            };
         });
     }