Adds test

Adds changelog entry

Change airbnb eslint config package to `eslint-config-airbnb-base` and update plugins.

Change `airbnb` to `airbnb-base` for .eslintrc `extends` value.

Added changelog entry

Made sure all plugins and envs are set

Corrected new failing specs

Replace static fixture for zen_mode_spec

## What does this MR do?

Replace `zen_mode.html.haml` for `zen_mode_spec.js` by already existing dynamically generated fixture.

## What are the relevant issue numbers?

#24753

See merge request !7686

Add `.find` poly

## What does this MR do?

Adds `[].find()` poly

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

## Why was this MR needed?

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) 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
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7710

Create dynamic fixture for build_spec

## What does this MR do?

Replace `spec/javascripts/fixtures/build.html.haml` by a dynamically created fixture (using `rake teaspoon:fixtures`).

## Why was this MR needed?

The existing fixture was not representing the real page. 

## What are the relevant issue numbers?

#24614 would have been avoided

following !6059

See merge request !7589