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/uploads_spec.rb

@@ -36,12 +36,21 @@ describe "Uploading files in chat messages", type: :system, js: true do
 
     it "allows uploading multiple files" do
       chat.visit_channel(channel_1)
+
       file_path_1 = file_from_fixtures("logo.png", "images").path
-      file_path_2 = file_from_fixtures("logo.jpg", "images").path
-      attach_file([file_path_1, file_path_2]) do
+      attach_file([file_path_1]) do
         channel.open_action_menu
         channel.click_action_button("chat-upload-btn")
+        find(".chat-composer-input").click
       end
+
+      file_path_2 = file_from_fixtures("logo.jpg", "images").path
+      attach_file([file_path_2]) do
+        channel.open_action_menu
+        channel.click_action_button("chat-upload-btn")
+        find(".chat-composer-input").click
+      end
+
       expect(page).to have_css(".chat-composer-upload .preview .preview-img", count: 2)
       channel.send_message("upload testing")