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-retention-reminder-test.js

@@ -14,9 +14,7 @@ module(
       this.channel = ChatChannel.create({ chatable_type: "Category" });
       this.currentUser.set("needs_channel_retention_reminder", true);
 
-      await render(
-        hbs`<ChatRetentionReminder @chatChannel={{this.channel}} />`
-      );
+      await render(hbs`<ChatRetentionReminder @channel={{this.channel}} />`);
 
       assert.dom(".chat-retention-reminder").includesText(
         I18n.t("chat.retention_reminders.public", {