Fixed anchoring diff lines in collapsed diffs

## What does this MR do?

This MR adds the ability to `forceLoad` a collapsed diff from instantiation, on top of this, if it is force loaded, we can pass a callback function to be invoked when the content is loaded.

Now when a MR is loaded, we check for a diff line anchor that is currently within a collapsed diff, we re-instantiate the `singleFileDiff` with the `forceLoad` option. We also pass a callback function that in responsible for highlighting and scrolling to the specified anchor.

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

## Why was this MR needed?

We currently cannot anchor link to collapsed diff lines as the do not auto-expand.

## Screenshots (if relevant)

https://youtu.be/fZVLlP6kchw

## Does this MR meet the acceptance criteria?

- [ ] [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
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #21633

See merge request !6285