DEV: rework the chat-live-pane

This PR is introducing glimmer usage in the chat-live-pane, for components but also for models. RestModel usage has been dropped in favor of native classes.

Other changes/additions in this PR:

- sticky dates, scrolling will now keep the date separator of the current section at the top of the screen
- better unread management, marking a channel as unread will correctly mark the correct message and not mark the whole channel as read. Tracking state will also now correctly return unread count and unread mentions.
- adds an animation on bottom arrow
- better scrolling behavior, we should now always correctly keep the scroll position while loading more
- reactions are now more reactive, and will update their tooltip without needed to close/reopen it
- skeleton has been improved with placeholder images and reactions
- when making a reaction on the desktop message actions, the menu won't move anymore
- simplify logic and stop maintaining a list of unloaded messages

plugins/chat/test/javascripts/components/chat-message-avatar-test.js

@@ -3,12 +3,16 @@ import hbs from "htmlbars-inline-precompile";
 import { exists, query } from "discourse/tests/helpers/qunit-helpers";
 import { module, test } from "qunit";
 import { render } from "@ember/test-helpers";
+import ChatMessage from "discourse/plugins/chat/discourse/models/chat-message";
+import fabricators from "../helpers/fabricators";
 
 module("Discourse Chat | Component | chat-message-avatar", function (hooks) {
   setupRenderingTest(hooks);
 
   test("chat_webhook_event", async function (assert) {
-    this.set("message", { chat_webhook_event: { emoji: ":heart:" } });
+    this.message = ChatMessage.create(fabricators.chatChannel(), {
+      chat_webhook_event: { emoji: ":heart:" },
+    });
 
     await render(hbs`<ChatMessageAvatar @message={{this.message}} />`);
 
@@ -16,7 +20,9 @@ module("Discourse Chat | Component | chat-message-avatar", function (hooks) {
   });
 
   test("user", async function (assert) {
-    this.set("message", { user: { username: "discobot" } });
+    this.message = ChatMessage.create(fabricators.chatChannel(), {
+      user: { username: "discobot" },
+    });
 
     await render(hbs`<ChatMessageAvatar @message={{this.message}} />`);