Merge branch 'eslint' into 'master'

Add ESLint

## What does this MR do?

- create `package.json`
- add ESLint dependencies to `package.json`
- add JavaScript linting to CI
- add Rake task `lint:javascript`as alias for `eslint` (which itself is an alias for `npm run eslint`)

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

Probably not.

## Why was this MR needed?

My hovercraft was full of eels.

## What are the relevant issue numbers?

- https://gitlab.com/gitlab-org/gitlab-ce/issues/13224#note_12537431
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5006#note_13255658


See merge request !5445

app/assets/javascripts/diff_notes/components/jump_to_discussion.js.es6

+/* eslint-disable */
 (() => {
   JumpToDiscussion = Vue.extend({
     mixins: [DiscussionMixins],

app/assets/javascripts/diff_notes/components/resolve_btn.js.es6

+/* eslint-disable */
 ((w) => {
   w.ResolveBtn = Vue.extend({
     props: {

app/assets/javascripts/diff_notes/components/resolve_count.js.es6

+/* eslint-disable */
 ((w) => {
   w.ResolveCount = Vue.extend({
     mixins: [DiscussionMixins],

app/assets/javascripts/diff_notes/components/resolve_discussion_btn.js.es6

+/* eslint-disable */
 ((w) => {
   w.ResolveDiscussionBtn = Vue.extend({
     props: {

app/assets/javascripts/diff_notes/diff_notes_bundle.js.es6

+/* eslint-disable */
 //= require vue
 //= require vue-resource
 //= require_directory ./models

app/assets/javascripts/diff_notes/mixins/discussion.js.es6

+/* eslint-disable */
 ((w) => {
   w.DiscussionMixins = {
     computed: {

app/assets/javascripts/diff_notes/models/discussion.js.es6

+/* eslint-disable */
 class DiscussionModel {
   constructor (discussionId) {
     this.id = discussionId;

app/assets/javascripts/diff_notes/models/note.js.es6

+/* eslint-disable */
 class NoteModel {
   constructor (discussionId, noteId, canResolve, resolved, resolved_by) {
     this.discussionId = discussionId;

app/assets/javascripts/diff_notes/services/resolve.js.es6

+/* eslint-disable */
 ((w) => {
   class ResolveServiceClass {
     constructor() {

app/assets/javascripts/diff_notes/stores/comments.js.es6

+/* eslint-disable */
 ((w) => {
   w.CommentsStore = {
     state: {},

app/assets/javascripts/dispatcher.js.es6

+/* eslint-disable */
 (function() {
   var Dispatcher;

app/assets/javascripts/dropzone_input.js

+/* eslint-disable */
 /*= require preview_markdown */

app/assets/javascripts/due_date_select.js.es6

+/* eslint-disable */
 (function(global) {
   class DueDateSelect {
     constructor({ $dropdown, $loading } = {}) {

app/assets/javascripts/extensions/array.js

+/* eslint-disable */
 Array.prototype.first = function() {
   return this[0];
 }

app/assets/javascripts/extensions/element.js.es6

+/* eslint-disable */
 Element.prototype.matches = Element.prototype.matches || Element.prototype.msMatches;
 Element.prototype.closest = function closest(selector, selectedElement = this) {

app/assets/javascripts/extensions/jquery.js

+/* eslint-disable */
 // Disable an element and add the 'disabled' Bootstrap class
 (function() {
   $.fn.extend({

app/assets/javascripts/files_comment_button.js

+/* eslint-disable */
 (function() {
   var bind = function(fn, me){ return function(){ return fn.apply(me, arguments); }; };

app/assets/javascripts/flash.js

+/* eslint-disable */
 (function() {
   this.Flash = (function() {
     var hideFlash;

app/assets/javascripts/gfm_auto_complete.js.es6

+/* eslint-disable */
 // Creates the variables for setting up GFM auto-completion
 (function() {
   if (window.GitLab == null) {

app/assets/javascripts/gl_dropdown.js

+/* eslint-disable */
 (function() {
   var GitLabDropdown, GitLabDropdownFilter, GitLabDropdownRemote,
     bind = function(fn, me){ return function(){ return fn.apply(me, arguments); }; },