PERF: applies optimisations on chat-live pane (#20532)

- group writes when computing separators positions
- shows skeleton only on initial load
- forces date separator to be pinned when first message to prevent a pinned - not pinned - pinned sequence when loading more in past
- relies on `message.visible` property instead of checking `isElementInViewport`
- attempts to load next/prev messages earlier
- do not scroll to on fetch more
- hides `last visit` text while pinned

plugins/chat/app/controllers/chat_controller.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class Chat::ChatController < Chat::ChatBaseController
-  PAST_MESSAGE_LIMIT = 20
+  PAST_MESSAGE_LIMIT = 40
   FUTURE_MESSAGE_LIMIT = 40
   PAST = "past"
   FUTURE = "future"