FIX: Pass ajax function to loadOneboxes (#12199)

When enable_diffhtml_preview is true, oneboxes failed to load because a
wrong parameter was passed.

This commit also unhides enable_diffhtml_preview.

app/assets/javascripts/discourse/app/components/d-editor.js

@@ -1,3 +1,4 @@
+import { ajax } from "discourse/lib/ajax";
 import {
   caretPosition,
   clipboardHelpers,
@@ -404,7 +405,7 @@ export default Component.extend({
         resolveCachedShortUrls(this.siteSettings, cookedElement);
         loadOneboxes(
           cookedElement,
-          null,
+          ajax,
           null,
           null,
           this.siteSettings.max_oneboxes_per_post,

config/site_settings.yml

@@ -977,7 +977,6 @@ posting:
     default: false
     client: true
   enable_diffhtml_preview:
-    hidden: true
     default: false
     client: true
   old_post_notice_days: