Merge branch '21369-make-it-clearer-that-diffs-can-be-collapsed' into 'master'

Updated diff toggle targets and added icon

## What does this MR do?

Adds the new diff toggle icon and alters the toggle targets.

User can now click on the file header bar _(where no other elements are above it, apart from the icon)_ and the `Click to expand` link to expand the diff and no where else.

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

## Why was this MR needed?

## Screenshots (if relevant)

![Screen_Shot_2016-09-02_at_15.35.15](/uploads/c1cb8c0547328153250294d6c95dd96a/Screen_Shot_2016-09-02_at_15.35.15.png)

####  Gif

![2016-09-02_15.34.31](/uploads/abaefaeba9ce8ef129522dae34574c57/2016-09-02_15.34.31.gif)

## 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 #21369 

Closes #20326

See merge request !6183

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