Replace static fixture for abuse_reports_spec

## What does this MR do?

Replace static HAML fixture for `abuse_reports_spec` by dynamically created one.

## What are the relevant issue numbers?

#24753

See merge request !7644

Add Object.assign polyfill

Because PhantomJS

See merge request !8090

Explicitly declare all javascript globals and all eslint rule violations

This merge request takes every single external global variable referenced within a javascript file and explicitly marks it with a `/* global Foo */` comment block at the top of the script.

This also expands all blanket instances of `/* eslint-disable */` with an explicit list of disabled rules.  This is useful because if we need to search for violations of a particular rule we can simply grep the codebase for something like `no-unused-vars` or `semi` and find all of the places where this rule has yet to be fixed.

Lastly, it also removes and resolves any existing `no-undef` eslint violations.  This is useful for catching mistakes like forgetting to declare a variable with `var`/`let`/`const` which can cause hard to find bugs.

## What does this MR do?

1. Looks for generic uses of `/* eslint-disable */` and refactors them into individual rule exceptions.
2. Looks for all occurrences of `/* eslint-disable ... no-undef */` and resolves them by either fixing bugs or declaring globals with `/* global Foo */`.

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

This touches a lot of files, most changes touch nothing other than comment blocks or whitespace.  The exceptions are the following 14 files which required some small bug fixes after removing `no-undef`:

- api.js
- breakpoints.js
- build.js
- commits.js
- diff_notes/components/jump_to_discussion.js.es6
- gfm_auto_complete.js.es6
- gl_dropdown.js
- groups_select.js
- importer_status.js
- namespace_select.js
- notes.js
- preview_markdown.js
- projects_list.js
- single_file_diff.js

## Why was this MR needed?

Removal of ~"technical debt" and some necessary changes to help !7288 

## Screenshots (if relevant)

N/A

## 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~~
  - [ ] 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?

N/A

See merge request !8043

Adds tests for Custom Event polyfill

## What does this MR do?
Adds tests for CustomEvent polyfill.

## Does this MR meet the acceptance criteria?

- [x] [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
  - [x] 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?
Closes #24927

See merge request !7996