Automatically expand hidden discussions when accessed via a permalink hash (closes #19304)

## What does this MR do?

Addresses "Auto-expand a discussion note when we arrive via anchor" (issue #19304)

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

Let me know if the code is consistent with community standards (not 100% sure about indentation of chained jQuery actions)

I don't think this MR warrants any unit tests or documentation, but let me know if you disagree.  I'll add the requisite CHANGELOG entry upon approval.

## Why was this MR needed?

Currently, if a discussion is accessed via a permalink to an "outdated" diff discussion, the anchor will not bring you to the expected part of the page.

## What are the relevant issue numbers?

#19304 

## 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)
- Tests
  - [ ] 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 !5585