Fix diff comments inverted toggle bug

## What does this MR do?
Explicitly set the comments visibility toggle based on the `active` css class.

## Are there points in the code the reviewer needs to double check?
Shouldn't be

## Why was this MR needed?
Fixes existing UI bug

## What are the relevant issue numbers?
Closes #20898 

## Screenshots (if relevant)
Before:
![A1ccIDEWsI](/uploads/21175e2008ee4e00225c22afebd873aa/A1ccIDEWsI.gif)

After:
![jHoFp69mAK](/uploads/e85f429d60a4e56d588faec51cd8648a/jHoFp69mAK.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [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 !5818