Keep edited timestamp when reacting to old notes

Copies updated_at to last_edited_at before bumping updated_at.

last_edited_at was added later and we decided not to do a backfill migration so we just did a fallback to updated_at for old notes.

Testing locally

1. Create and edit a comment
2. Manually clear the last_edited_at of the note. (Note.last.update_column(:last_edited_at, nil))
3. Add an emoji to the note
4. Wait for the poll or refresh the page

Related to #349040 (closed)