Merge branch 'custom-event-polyfill' into 'master'

Adds polyfill for CustomEvent

## What does this MR do?
CustomEvent is not supported by IE.- https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent

In order to stop using jQuery in new files that need to listen to custom events we need this polyfill.

## 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
  - [ ] 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)
- [ ] [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 !7705

app/assets/javascripts/lib/utils/custom_event_polyfill.js.es6

+/**
+ * CustomEvent support for IE
+ */
+if (typeof window.CustomEvent !== 'function') {
+  window.CustomEvent = function CustomEvent(e, params) {
+    const options = params || { bubbles: false, cancelable: false, detail: undefined };
+    const evt = document.createEvent('CustomEvent');
+    evt.initCustomEvent(e, options.bubbles, options.cancelable, options.detail);
+    return evt;
+  };
+  window.CustomEvent.prototype = window.Event.prototype;
+}