Load the org.xwiki.platform.html.head UIXP in the editor head

Loading the org.xwiki.platform.html.head UIXP in the editor head will allow to have access to some skin resources, even after the editor is re-rendered (for instance after a macro is inserted).

See

• https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/ExtensionPoint/HTMLHead
• https://github.com/xwiki-contrib/application-ckeditor/blob/master/application-ckeditor-ui/src/main/resources/CKEditor/ContentSheet.xml