Merge branch 'fix-linting' into 'master'

Enable linting for ES6 files

## What does this MR do?

Add `.js.es6` to file extensions checked by ESLint.

## What are the relevant issue numbers?

Follows !5445

See merge request !7156

app/assets/javascripts/boards/components/board_sidebar.js.es6

+/* eslint-disable */
 (() => {
   const Store = gl.issueBoards.BoardsStore;
 

app/assets/javascripts/boards/filters/due_date_filters.js.es6

+/* eslint-disable */
 Vue.filter('due-date', (value) => {
   const date = new Date(value);
   return $.datepicker.formatDate('M d, yy', date);

app/assets/javascripts/boards/models/milestone.js.es6

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

app/assets/javascripts/header.js

+/* eslint-disable */
 (function() {
 
   $(document).on('todo:toggle', function(e, count) {

package.json

 {
   "private": true,
   "scripts": {
-    "eslint": "eslint .",
-    "eslint-fix": "eslint --fix ."
+    "eslint": "eslint --ext .js,.js.es6 .",
+    "eslint-fix": "eslint --fix --ext .js,.js.es6 ."
   },
   "devDependencies": {
     "eslint": "^3.1.1",

spec/javascripts/dashboard_spec.js.es6

+/* eslint-disable */
 /*= require sidebar */
 /*= require jquery */
 /*= require jquery.cookie */

spec/javascripts/header_spec.js

+/* eslint-disable */
 /*= require header */
 /*= require lib/utils/text_utility */
 /*= require jquery */