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/spec/system/flag_message_spec.rb

@@ -32,7 +32,7 @@ RSpec.describe "Flag message", type: :system, js: true do
 
   context "when direct message channel" do
     fab!(:dm_channel_1) { Fabricate(:direct_message_channel, users: [current_user]) }
-    fab!(:message_1) { Fabricate(:chat_message, chat_channel: dm_channel_1, user: current_user) }
+    fab!(:message_1) { Fabricate(:chat_message, chat_channel: dm_channel_1) }
 
     it "doesn’t allow to flag a message" do
       chat.visit_channel(dm_channel_1)