DEV: no need to track status of a deleted user (#22836)

It is hard to catch and debug potential bugs related to live updates of user status 
(though, we haven't seen many such bugs so far). We have a `console.warn` 
statement that should help us to catch one class of such bugs.

Recently, we noticed that this warning gets printed when a user had a chat with 
a user that was then deleted.

This is not a bug, since there is nothing to track for a deleted user, but we don't 
want this noise on the console. This PR makes sure we don't print a warning in 
such cases.

plugins/chat/assets/javascripts/discourse/initializers/chat-sidebar.js

@@ -210,7 +210,10 @@ export default {
               this.chatService = chatService;
 
               if (this.oneOnOneMessage) {
-                this.channel.chatable.users[0].trackStatus();
+                const user = this.channel.chatable.users[0];
+                if (user.username !== I18n.t("chat.deleted_chat_username")) {
+                  user.trackStatus();
+                }
               }
             }
 

plugins/chat/config/locales/client.en.yml

@@ -25,6 +25,7 @@ en:
       chat_users_count: "Chat Users"
 
     chat:
+      deleted_chat_username: deleted
       dates:
         time_tiny: "h:mm"
       all_loaded: "Showing all messages"