Fix flickering of system notes

What does this MR do?

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1873

• Fix flickering of system notes
  • Add original note content to compare against

Are there points in the code the reviewer needs to double check?

• Do we want this in 9.2?
  • Note edit updates was added in 9.2, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10837

Easy reproduction:

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

Why was this MR needed?

• System notes flickering after being added

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #32087 (closed)