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-separator-new-test.js

@@ -0,0 +1,24 @@
+import { setupRenderingTest } from "discourse/tests/helpers/component-test";
+import { query } from "discourse/tests/helpers/qunit-helpers";
+import hbs from "htmlbars-inline-precompile";
+import I18n from "I18n";
+import { module, test } from "qunit";
+import { render } from "@ember/test-helpers";
+
+module(
+  "Discourse Chat | Component | chat-message-separator-new",
+  function (hooks) {
+    setupRenderingTest(hooks);
+
+    test("newest message", async function (assert) {
+      this.set("message", { newest: true });
+
+      await render(hbs`<ChatMessageSeparatorNew @message={{this.message}} />`);
+
+      assert.strictEqual(
+        query(".chat-message-separator-new").innerText.trim(),
+        I18n.t("chat.last_visit")
+      );
+    });
+  }
+);