Optimize cross ref system notes check

## What does this MR do?

This MR optimizes system note visibility checking by memoizing the visible reference count, reducing the overhead of calling `Note#cross_reference_not_visible_for?`.

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

Note that since a cross reference message contains, "Mentioned in XYZ", we EXPECT that there is at least one reference. That's why using the ref count > 0 works. 

## Why was this MR needed?

The previous implementation relied on `Note#cross_reference_not_visible_for?`, which essentially tries to render all the Markdown references in a system note
and only displays the note if the user can see the referring project. But this duplicated the work that Banzai::NotesRenderer was doing already. This shaves about 0.8 s
from the load time from https://gitlab.com/gitlab-com/operations/issues/42.

## What are the relevant issue numbers?

#19273

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5070