Fix diff comments not showing up in activity feed

## What does this MR do?

It fixes the detection of note events to check for `Note` and `LegacyDiffNote`.

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

No? /cc @DouweM (since I believe you introduced `LegacyDiffNote`

## Why was this MR needed?

To fix #19092.

## What are the relevant issue numbers?

Fixes #19092.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [ ] 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 !5069