DEV: rework the chat-live-pane (#20511)

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-channel-row-test.js

@@ -51,9 +51,7 @@ module("Discourse Chat | Component | chat-channel-row", function (hooks) {
 
     assert
       .dom(".chat-channel-metadata")
-      .hasText(
-        moment(this.categoryChatChannel.last_message_sent_at).format("l")
-      );
+      .hasText(moment(this.categoryChatChannel.lastMessageSentAt).format("l"));
   });
 
   test("renders membership toggling button when necessary", async function (assert) {