Merge branch '24492-promise-polyfill' into 'master'

Adds es6-promise polyfill

## What does this MR do?
Adds [es6-promise](https://github.com/stefanpenner/es6-promise) 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
  - [ ] 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 #24492

See merge request !7482

app/assets/javascripts/application.js

@@ -53,6 +53,7 @@
 /*= require_directory ./u2f */
 /*= require_directory . */
 /*= require fuzzaldrin-plus */
+/*= require es6-promise.auto */
 
 (function () {
   document.addEventListener('page:fetch', gl.utils.cleanupBeforeFetch);

changelogs/unreleased/24492-promise-polyfill.yml

+---
+title: Adds es6-promise Polyfill
+merge_request: 7482
+author: