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/serializer/chat_message_serializer_spec.rb

@@ -21,12 +21,14 @@ describe ChatMessageSerializer do
       it "doesn’t return the reaction" do
         Emoji.clear_cache
 
-        expect(subject.as_json[:reactions]["trout"]).to be_present
+        trout_reaction = subject.as_json[:reactions].find { |r| r[:emoji] == "trout" }
+        expect(trout_reaction).to be_present
 
         custom_emoji.destroy!
         Emoji.clear_cache
 
-        expect(subject.as_json[:reactions]["trout"]).to_not be_present
+        trout_reaction = subject.as_json[:reactions].find { |r| r[:emoji] == "trout" }
+        expect(trout_reaction).to_not be_present
       end
     end
   end