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/spec/system/navigating_to_message_spec.rb

@@ -60,8 +60,9 @@ RSpec.describe "Navigating to message", type: :system, js: true do
 
       it "highlights the correct message after using the bottom arrow" do
         chat_page.visit_channel(channel_1)
+
         click_link(link)
-        click_link(I18n.t("js.chat.scroll_to_bottom"))
+        click_button(class: "chat-scroll-to-bottom")
         click_link(link)
 
         expect(page).to have_css(
@@ -149,8 +150,9 @@ RSpec.describe "Navigating to message", type: :system, js: true do
         visit("/")
         chat_page.open_from_header
         chat_drawer_page.open_channel(channel_1)
+
         click_link(link)
-        click_link(I18n.t("js.chat.scroll_to_bottom"))
+        click_button(class: "chat-scroll-to-bottom")
         click_link(link)
 
         expect(page).to have_css(