DEV: Use UploadReference instead of ChatUpload in chat (#19947)

We've had the UploadReference table for some time now in core,
but it was added after ChatUpload was and chat was just never
moved over to this new system.

This commit changes all chat code dealing with uploads to create/
update/delete/query UploadReference records instead of ChatUpload
records for consistency. At a later date we will drop the ChatUpload
table, but for now keeping it for data backup.

The migration + post migration are the same, we need both in case
any chat uploads are added/removed during deploy.

plugins/chat/lib/chat_transcript_service.rb

@@ -147,7 +147,7 @@ class ChatTranscriptService
   def messages
     @messages ||=
       ChatMessage
-        .includes(:user, chat_uploads: :upload)
+        .includes(:user, upload_references: :upload)
         .where(id: @message_ids, chat_channel_id: @channel.id)
         .order(:created_at)
   end