Filter out system notes referencing inaccessible work items

What does this MR do and why?

Filter out system notes referencing inaccessible work items

Usually if there are system notes that reference an item that cannot be accessed by the user we filter out that system note altogether.

This was not happening for system notes about adding or removing a child work item or a parent work item.

re https://gitlab.com/gitlab-org/gitlab/-/issues/471981

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Adding the screenshots from the fix here in the meantime, so that I can then copy these over to the MR itself:

Page	Member	Anonymous(before)	Anonymous(after)
public issue linking to an epic in a public group(/-/issues page)
same issue as above( /-/work_items page)
public epic that was linked to the issue above(/-/epics page)
same epic as above(/-/work_items page)
public issue linking to an epic in a private group(/-/issues page)
public issue linking to an epic in a private group(/-/work_items page)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.