DEV: Warn when trying to track user status for user models without ID (#21205)

User status updates come from the server in a map where keys are user IDs. 
If user.trackStatus() is called for a user model without an ID, the model 
cannot identify its status updates and silently misses them. It's quite hard to 
notice that a user rendered in the UI doesn't receive live status updates. 
Also, it's not immediately obvious what's the reason of the problem. 
A warning will be very helpful here.

app/assets/javascripts/discourse/app/models/user.js

@@ -1401,6 +1401,13 @@ User.reopen(Evented, {
 
   // always call stopTrackingStatus() when done with a user
   trackStatus() {
+    if (!this.id) {
+      // eslint-disable-next-line no-console
+      console.warn(
+        "It's impossible to track user status on a user model that doesn't have id. This user model won't be receiving live user status updates."
+      );
+    }
+
     if (this._subscribersCount === 0) {
       this.addObserver("status", this, "_statusChanged");