DEV: fix flakeyness with drawer specs (#22476)

Chat drawer was using the `DiscourseURL` hook `afterRouteComplete`. This hook suffer from a very poor implementation which makes it very unreliable:

```javascript
if (typeof opts.afterRouteComplete === "function") {
  schedule("afterRender", opts.afterRouteComplete);
}
```

This commit attempts to return the promise from `handleURL` to directly use it and have a very reliable after transition hook.

app/assets/javascripts/discourse/app/lib/url.js

@@ -464,7 +464,7 @@ const DiscourseURL = EmberObject.extend({
     transition._discourse_original_url = path;
 
     const promise = transition.promise || transition;
-    promise.then(() => this.jumpToElement(elementId));
+    return promise.then(() => this.jumpToElement(elementId));
   },
 
   jumpToElement(elementId) {

plugins/chat/assets/javascripts/discourse/components/chat-channel.js

@@ -825,13 +825,8 @@ export default class ChatLivePane extends Component {
   onCloseFullScreen() {
     this.chatStateManager.prefersDrawer();
 
-    DiscourseURL.routeTo(this.chatStateManager.lastKnownAppURL, {
-      afterRouteComplete: () => {
-        this.appEvents.trigger(
-          "chat:open-url",
-          this.chatStateManager.lastKnownChatURL
-        );
-      },
+    DiscourseURL.routeTo(this.chatStateManager.lastKnownAppURL).then(() => {
+      DiscourseURL.routeTo(this.chatStateManager.lastKnownChatURL);
     });
   }
 

plugins/chat/spec/system/navigation_spec.rb

@@ -120,7 +120,7 @@ RSpec.describe "Navigation", type: :system do
   end
 
   context "when collapsing full page with previous state" do
-    xit "redirects to previous state" do
+    it "redirects to previous state" do
       visit("/t/-/#{topic.id}")
       chat_page.open_from_header
       chat_drawer_page.maximize