UX: implements draft threads (#21361)

This commit implements all the necessary logic to create thread seamlessly. For this it relies on the same logic used for messages and generates a `staged-id`(using the format: `staged-thread-CHANNEL_ID-MESSAGE_ID` which is used to re-conciliate state client sides once the thread has been persisted on the backend.

Part of this change the client side is now always using real thread and channel objects instead of sometimes relying on a flat `threadId` or `channelId`.

This PR also brings three UX changes:
- thread starts from top
- number of buttons on message actions is dependent of the width of the enclosing container
- <kbd>shift + ArrowUp</kbd> will reply to the last message

plugins/chat/spec/system/page_objects/chat_drawer/chat_drawer.rb

@@ -31,8 +31,12 @@ module PageObjects
         find("#{VISIBLE_DRAWER} .chat-drawer-header__full-screen-btn").click
       end
 
-      def has_open_thread?(thread)
-        has_css?("#{VISIBLE_DRAWER} .chat-thread[data-id='#{thread.id}']")
+      def has_open_thread?(thread = nil)
+        if thread
+          has_css?("#{VISIBLE_DRAWER} .chat-thread[data-id='#{thread.id}']")
+        else
+          has_css?("#{VISIBLE_DRAWER} .chat-thread")
+        end
       end
 
       def has_open_channel?(channel)