Support WordPress 4.9

[pierlon]

Summary

Ensures plugin works on WordPress 4.9.

Checklist

• [x] My pull request is addressing an open issue (please create one otherwise).
• [ ] My code is tested and passes existing tests.
• [x] My code follows the Contributing Guidelines (updates are often made to the guidelines, check it out periodically).

[spacedmonkey]

Apifetch is only used by the importImage. It is would be easier to just replace with the apirequest library. We use this elsewhere. ApiRequest seems to have been added in 4.9, so we can use it.

Thoughts?

Otherwise, I like breaking off the gutenberg only override into other file.

[pierlon]

Apifetch is only used by the importImage. It is would be easier to just replace with the apirequest library. We use this elsewhere. ApiRequest seems to have been added in 4.9, so we can use it.

The only downside I see would be losing out on the Promise it returns. We could just use window.fetch instead, but would we need a polyfill for that? Not sure if we need to accomodate older browsers such as IE.

[spacedmonkey]

The only downside I see would be losing out on the Promise it returns

jQuery ajax doesn't use native browser promises, but it does use a javascript version of promises, which I think is good enough here.

It would also be super simple to wrap apiRequest it in a native promise. Something like this.

function ajax(options) {
  return new Promise(function (resolve, reject) {
    $.ajax(options).done(resolve).fail(reject);
  });
}

See this.

We could just use window.fetch instead, but would we need a polyfill for that? It is much simpler, faster and requires less testing to use an existing apiRequest package.

[spacedmonkey]

Tests are broken.