Fix rebase in IE

What does this MR do?

• Fix rebase in IE
• Switch to using Axios in EE MR widget utilities

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

• We needed to add X-Requested-With: XMLHttpRequest header to the Axios defaults because Rails will just 404 and not consider our request. Head-banging issue to figure out 😩
  • See Rails source, https://github.com/rails/rails/blob/38b2904f2c9d89868e96edd71c5f3143ca235a88/actionpack/lib/action_dispatch/http/request.rb#L267
  • Related posts
    • https://laracasts.com/discuss/channels/vue/laravel-not-detecting-ajax-request-when-made-with-vue-axios
    • https://github.com/axios/axios/issues/891#issuecomment-343197626
    • https://stackoverflow.com/a/8220761/796832
    • https://stackoverflow.com/q/17478731/796832
    • https://github.com/rails/rails/issues/8987

---

It looks like https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12658 didn't actually solve https://gitlab.com/gitlab-org/gitlab-ce/issues/34534. vue-resource@1.3.4 still sends undefined in the POST body which in IE means it will literally send undefined that causes the BE to hiccup. This also means the other areas are still broken.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #3501 (closed)