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/helpers/fabricators.js

@@ -3,6 +3,7 @@ import ChatChannel, {
 } from "discourse/plugins/chat/discourse/models/chat-channel";
 import EmberObject from "@ember/object";
 import { Fabricator } from "./fabricator";
+import ChatMessage from "discourse/plugins/chat/discourse/models/chat-message";
 
 const userFabricator = Fabricator(EmberObject, {
   id: 1,
@@ -38,7 +39,7 @@ export default {
     },
   }),
 
-  chatChannelMessage: Fabricator(EmberObject, {
+  chatChannelMessage: Fabricator(ChatMessage, {
     id: 1,
     chat_channel_id: 1,
     user_id: 1,