Add metadata for system notes

What does this MR do?

• adds model for storing metadata for system notes (SystemNoteMetadata)
• creates a metadata record when a system note is created

Why was this MR needed?

We need metadata for displaying icons for system notes (#24784 (closed)) and it is alsi needed for #19836 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • 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?

#24784 (closed)

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1526