Fix flickering of system notes

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32087

Easy reproduction:

 1. Edit an issue title
 1. Notice the system note added
 1. Switch to another tab and back (to fire the polling immediately)
 1. Notice the flicker (without this fix)

app/views/shared/notes/_note.html.haml

@@ -29,6 +29,8 @@
               - if note.system
                 %span.system-note-message
                   = note.redacted_note_html
+                .original-note-content.hidden
+                  = note.note
               %a{ href: "##{dom_id(note)}" }
                 = time_ago_with_tooltip(note.created_at, placement: 'bottom', html_class: 'note-created-ago')
         - unless note.system?