Toggle whitespace button for compare page

## What does this MR do?

Adds whitespace button to compare branches page.

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

Ruby code.

## Why was this MR needed?

Compare branches UX

## What are the relevant issue numbers?

Closes #17881.

## Screenshots (if relevant)

![Screen_Shot_2016-06-09_at_18.39.48](/uploads/8995a7bc8bea7fa3ff19446ebe49fb88/Screen_Shot_2016-06-09_at_18.39.48.png)

## 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)
- [ ] API support added
- [ ] 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 !4569