FIX: Better handling of deleted thread original messages (#22402)

This commit includes several fixes and improvements to thread
original message handling:

1. When a thread's original message is deleted, the thread no longer
   counts as unread for a user
2. When a thread original message is deleted and the user is looking
   at the thread list, it will be removed from the list
3. When a thread original message is restored and the user is looking
   at the thread list, it will be added back to the list if it was
   previously loaded

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

@@ -58,6 +58,10 @@ module PageObjects
         has_css?(".chat-selection-management")
       end
 
+      def expand_deleted_message(message)
+        message_by_id(message.id).find(".chat-message-expand").click
+      end
+
       def expand_message_actions(message)
         hover_message(message)
         click_more_button

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

@@ -132,10 +132,29 @@ module PageObjects
         end
       end
 
+      def expand_deleted_message(message)
+        message_by_id(message.id).find(".chat-message-expand").click
+      end
+
       def copy_link(message)
+        expand_message_actions(message)
+        find("[data-value='copyLink']").click
+      end
+
+      def delete_message(message)
+        expand_message_actions(message)
+        find("[data-value='delete']").click
+      end
+
+      def restore_message(message)
+        expand_deleted_message(message)
+        expand_message_actions(message)
+        find("[data-value='restore']").click
+      end
+
+      def expand_message_actions(message)
         hover_message(message)
         click_more_button
-        find("[data-value='copyLink']").click
       end
 
       def click_more_button

plugins/chat/spec/system/page_objects/chat/components/thread_list.rb

@@ -23,6 +23,10 @@ module PageObjects
           item_by_id(thread.id)
         end
 
+        def has_no_thread?(thread)
+          component.has_no_css?(item_by_id_selector(thread.id))
+        end
+
         def item_by_id(id)
           component.find(item_by_id_selector(id))
         end